Pyrolytic cyclization

Cyclization of the urido-l,2,4-trizine derivatives (634) to the 1,2,4-triazino[2,3-c]quinazoline (635) by heating with polyphosphoric acid has already been discussed (see Section XVI.B.7) (74JHC747). However, pyrolytic cyclization of 634 by heating at 200° afforded the 1,2,4-triazino[4,3-c]quinazoline (638) as a result of eliminative cyclization between the urido function and N-4 of the 1,2,4-triazine ring (74JHC747). The structure of 638 was confirmed by an unequivocal synthesis from 2-oxo-4-thioxoquinazoline (639) (74JHC747). 

In place of a Grignard reagent, several homoenolate equivalents have also been employed. Kempt 1 7 reported the titanium-mediated addition of /V-alkylmethylacrylamide dianions to N-protected a-amino aldehydes (Scheme 8). Pyrolytic cyclization affords a 3-methylenetetrahydrofuran-2-one and the side chain of C3 is appended via conjugate addition. The resulting lactone can be converted into the 1-hydroxyethylene dipeptide by hydrolysis. The stereochemistry of the C6 atom is the same as that of the a-amino aldehyde. However, the stereoselectivities of the reactions regarding the C3 and C5 atoms are unsatisfactory. 

Syntheses from precursors of types (405) and (406) are rare, and of type (407) are unknown. One example of the first of these is the oxidative cyclization of pyrimidine (408) to the glycosyl pyrimidotriazine (409) (Equation (68)) <82MI 720-01). An interesting example of the second type is the cyclization of the 6-chloropyrimidine (411). In the presence of air this affords 2-methyl-fervenulone (412) by straightforward intramolecular nucleophilic substitution of the chlorine atom. Pyrolytic cyclization of compound (411), however, affords the isomeric 1-methyl product (413), possibly via a diaziridine intermediate (Scheme 33) <75JOC232i>. 

Activated copper powder or copper(II) acetylacetonate were effective catalysts in the pyrolytic cyclization of 2-azido-3-(2,4-pentadienyl)-l,4-quinones to benzopyrolizines 17 by the presumed intermediacy of a copper(II)-nitrene species which adds to the diene by a radical pathway. Although the yields were generally low, complete regioselectivity and diastereoselectivity (simple and substrate induced) were observed152. 

A number of reactions of metal salts can be rationalized in terms of the formation of a carbanion adjacent to the carboxylate. Dibasic metals such as calcium bring two carboxylate units close to each other so that the carbanion formed adjacent to one carboxylate may attack the carbonyl of the other. Thus pyrolysis of calcium acetate affords propanone (acetone) (Scheme 3.62). A similar reaction is found in the pyrolytic cyclization of some dicarboxylic acid anhydrides. Heating Cg and dicarboxylic acids gives cyclopentanones and cyclohexanones... 

As a step in the synthesis of a higher homologue of the antibiotic thienamycin, the closure of the pyridine ring is effected by warming the diazo ester (793) with rhodium(II) acetate (review of rhodium-catalysed reactions [3928]) the product (79.6, R = H) is more easily isolated and purified as its O-tosyl derivative (79.6, R = Ts). Intramolecular homolytk cyclization (the Pschorr reaction) of the diazonium salt of N-methylbenzanilide gives moderate yields which are comparable with those given by the pyrolytic cyclization of the 2-iodo analogue (see Chapter 90, Section II.2 [2303]). 

R2 = NH2 R8 = R4 = H). The hydrolysis of the latter affords 39 (Rl = R2 = H). Hydrogen iodide can also be used for the reaction however, hydrogen chloride is ineffective.20-22-38 The diamide (47) was pyrolytically cyclized at 295° into 39 (Rl = R2 = H) in good yield but in a high-boiling solvent only a poor yield was obtained.89... 

As alluded (vide supra), some confusion may arise with respect to this named reaction as there is reference in the literature to an alternative reaction with the same name. The Bradsher reaction forms aromatic rings but via an acid-catalyzed Friedel-Crafts-like process. Thus diaryl-methanes having a carbonyl group in the ortho position can undergo a cyclodehydration reaction to generate the corresponding anthracene derivatives. In this respect, the Bradsher reaction is related to the Elbs reaction, which involves the pyrolytic cyclization of diaryl ketones 6 having an ortho methyl or methylene substituent for the formation of polycyclic aromatics 7. 

Reactions of the double bonds include isomerization and conjugation, cyclization, various addition reactions including hydrogenation, pyrolytic and... 

As already discussed in Section 2.2, crystalline dimethylsilanediol 53 can be prepared by hydrolysis from hexamethylcyclotrisilazane 51, from dimethoxydimethyl-silane [40], and from octamethylcyclotetrasilazane (OMCTS) 52. The most simple preparation of 53 is, however, controlled hydrolysis of dimethyldichlorosilane 48 in the presence of (NH4)2C03 or triethylamine [41]. Likewise, hydrolysis of hexam-ethylcyclotrisiloxane 54 and of octamethylcyclotetrasiloxane 55 eventually gives rise to dimethylsilanediol 53. In all these reactions the intermediacy of the very reactive dimethylsilanone 110 has been assumed, which can be generated by pyrolytic [42, 43] and chemical methods [44—46] and which cyclizes or polymerizes much more rapidly, e.g. in contact with traces of alkali from ordinary laboratory or even Pyrex glassware [40, 47] to 54, 55, and 56 than trimethylsilanol 4 polymerizes to hexamethyldisiloxane 7. Compound 111 is readily converted into dimethylsilanone 110 and MesSil 17 [46] (Scheme 3.6). 

Isopulegol can be isolated from this mixture and hydrogenated to (-)-menthol. The remaining isopulegol stereoisomers can be partly reconverted into (+)-citronellal by pyrolytic cleavage and reused in the cyclization procedure [79]. 

Although the majority of studies in this area involve five-membered ring cyclic anhydrides, a few pyrolytic reactions involving acyclic anhydrides have been reported. Thus, for example, FVP of 278 gives the alkylideneketene 279 with loss of trifluoroacetic acid142, while at 650 °C 280 loses both trifluoroacetic acid and cyclopentadiene to afford the indenylideneketene 281, which cyclizes by way of 282 to give 283143. FVP of... 

The best general procedure for preparing 2-substituted phenox-azines is the pyrolytic condensation of o-chloronitrobenzene derivatives with sodium o-bromophenolate,20,29 followed by reduction with stannous chloride or with iron filings in acetic acid and subsequent cyclization of the ether 10. If the ether 10 is previously A-formylated,... 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

The condensation of the dilithio derivative of (R)-(+)-3-(p-tolylsulfinyl)propionic acid with protected glycoaldehy-des (O-r-butyl and 0-benzyl) gives 5-alkoxy-4-hydroxy-3-(p-tolylsulfinyl)pentanoic acids, which spontaneously cyclize to the corresponding 3-sulfinyl-4-alkoxymethyl butanolides (eq 4). Pure diastereomers can be separated by flash chromatography and are obtained in comparable amounts. The corresponding optically pure butenolides are obtained by pyrolytic elimination of the sulfoxides and then transformed into natural (-i-)-(/ )-umbelactone (eq5). 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

Finally, zearalenone was obtained by intramolecular alkylation of the carbanion a to the benzylic sulfide, which occurs readily with potassium hexamethyldisilazane. After cyclization, the double bond was created by oxidation of sulfide to sulfoxide followed by pyrolytic elimination, a technique described in more detail in the next section (Scheme 12). 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

In a recent study (120), measurements were made of the molar cyclization equilibrium constants Kx for cyclics (0(CH2)i00C0(CH2)4C0)x with x = 1—5 in an undiluted equilibrate of poly(decamethylene adipate) (PDA) at 423 K, and for cyclics (0(CH2)30C0(CH2 )2 CO)x with x = 1-7 in an undiluted equilibrate of poly(trimethylene succinate) (PTS) at the same temperature. The polymers were prepared from dimethyl adipate and decamethylene glycol and from dimethyl succinate and 1,3-propane diol using tetraisopropyltitanate and equilibrated at the required temperature in four-necked glass reaction kettles. Cyclics were extracted from the polymeric equilibrates and analysed by g.p.c. by methods described in Ref. (120). Individual cyclics were also prepared from the polyesters by the general pyrolytic method of Carothers and these were used for identification and calibration purposes. 

Pyrolytic heterocyclization of (214) led to (215) (Equation (54)), whereas treatment of (216) with aroyl chloride afforded the [l,3,4]thiadiazolo[3,2-c]pyrimidinium salt (217), a representative of a new ring system (Equation (55)) <87M485>. Compound (219), prepared from (218) (Equation (56)), also represents a new ring system, [l,2,4]thiadiazolo[4,3-c]pyrimidine <86S1027>, and similarly, formation of the ring system present in [l,2,4]thiadiazolo[2,3-c]pyrimidine (48) (cyclization of (220) to (221) (Equation (57)) <77JHC62l is also reported for the first time. 

Unsaturated polymers, particularly of dienic monomers, undergo a number of interesting thermal rearrangements under non-pyrolytic conditions. Golub has studied these reactions in some detail and has recently reviewed the subject. In the past two years accounts of thermal rearrangements of l,2-poly(hexa-l,4-diene)s and l,2-poly(/ra 5-penta-l,3-diene) have appeared. The former polymers have a predominantly 1,8-diene structure and cyclize mainly by a [2 + 2] or type II mechanism accompanied by a small amount of a type III reaction (Scheme 24). The latter is more important in the trans-, A- than in the cir-1,4-isomer. The first unambiguous example of a type III reaction was provided by the polymer of penta-1,3-diene. Scheme 25 shows a macromolecular... 

An analysis of the published data suggests that phosphorus flame retardants or products of their transformation serve as agents and catalysts for the substituent detachment reactions in the macromolecular chain, for cyclization reactions and for other reactions ofpolymers. As has already been mentioned, such reactions favor the formation of the carbon skeleton. Pyrolytic dehydrogenation reactions and dehydrohalogenation reactions of organic compounds are aided by the presence of phosphorus compounds. In this case, one of the most important factors to be considered is the relationship between the chemical structure of phosphorus-containing compounds and their reactivity, which determines the specific interaction with a polymer substrate under the conditions of its preliminary treatment and combustion. 

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