Open Chain Systems

1 Open-chain Systems - Chow and his coworkers have carried out further studies on the cycloaddition reactions of dibenzoyl(methanato)boron difluoride (1) to alkenes and dienes. This present work has examined the mechanistic details of the process. Previously they have reported the addition of the same reactant to enol ethers. The influence of the chiral centre in the alkene (2) on the cycloaddition of a,P-unsaturated nitriles such as acrylonitrile has been evaluated. The products obtained from these (2+2)-photoaddition reactions are the azabicyclooctanes (3).  

1 Open-chain Systems - Several reports over the last few years have made use of the pentenoate (1) in photochemical cycloaddition reactions. Typical of these 

Photochemical cycloaddition of dimethyl fumarate and dimethyl maleate to the psoralen (15) has been reported. The adducts formed are presumed to be of the (2+2)-type illustrated by (16) where addition to the furan ring has occurred. The photodimerization of a,to-bis(4-methylcoumarin)tetraethylene glycol is re-giospecific and only the syn head-to-tail dimer is formed.  

Further examples of the photoaddition of alkenes to the diketonatoboron difluoride (17) have been published. The irradiations are carried out in 1,4-dioxane or acetonitrile as solvent and use 350 nm light. Irradiation times are relatively long (20 h) but result in the formation of the expected adducts. Thus addition of the alkenes (18) affords the 1,5-diketones (19). These arise by ring opening of the initially formed cyclobutane adducts [e.g. (20)]. Similar results are 

The solid state dimerization of three polymorphic forms of (24) has been reported. From these irradiations, the dimer (25) is obtained when light in the range 320-400 nm is used. The principal product (25) is obtained in 58% yield and is accompanied by several minor products. Another report of the photochemical reactivity of (24) has shown that irradiation in solution brings about aromatiza-tion quantitatively.In the crystal, however, irradiation of the (4R ,TRS)-(24) affords the dimer (25). Apparently this dimerization in the crystalline phase is possible because of extra space within the crystal, but this is not so with crystals 

Intermolecular Additions.- Open Chain Systems. The involvement of a singlet exciplex is proposed in the selective photoaddition of 2,3-dimethylbut-2-ene to the /-auf-nitrile (38) yielding the (2- -2)-adducts (39). The photodimerization of (40) affords the cyclobutane derivative (41).  

The de Mayo addition resxtion continues to be of considerable value for the formation of precursors for further synthesis. One example of this is the formation of the photoadducts (42, 43), prepared by photoaddition of isoprene to the enol of the dione ester (44), and have been used in a general synthesis of cyclopentenes. The photoadducts (42, 43) are produced in the standard fashion by the formation of a labile cyclobutane which subsequently ring opens. Takeshita et al. have reported the 

Photoaddition of the ynone (47) to 1,1-diethoxyethene gave the oxetan (48) (50%), by a conventional (2+2)-cycloaddition route, as well as the furan derivative (49) (18%). The formation of this latter product arises by way of the carbene (50), itself a product of (3- -2)-addition of the ynone to the alkene. Intramolecular trapping of the carbene 

Cyclopentenones. Photo-(2+2)-cycloEwidition of 1,1-diethoxyethene to cyclopentenone yields the adduct (54). Subsequent involved chemical transformations were used to complete a total synthesis of racemic A -capaeJ/ene The cyclopentenone (55) photochemically ( a laser source) adds hex-l-yne to yield the cycloadduct (56) in good yield. By using a Pyrex-filtered Hg arc lamp the same starting materials yield the tropovalene (57) (25%) and the bicyclic product (56) (38%). Independent irradiation of the tropovalene (57) yields the tropone (58) (83%). The cycloadduct (56) can also be converted into the tropone (58) by AI2O3 catalyzed elimination of acetic acid followed by irradiation of the enone (59).  

Meyers and Fleming have reported a synthesis of -)-GrandisoI (60) conunencing with photochemical (2+2)-addition of ethene to the chiral enone (61). This yields the adduct (62) in 93% yield contaminated with 7-8% of the endb-isomer. Ring opening of the adduct (62) affords the two keto esters (63) and (64) in 45 and 55% respectively. The former is taken on to complete the synthesis of Grandisol  

The photochemistry of a phenyldiacrylic acid derivative has been studied in Langmuir-Blodgett films J The crystal structure of the major dimer formed on perdeuterioacetone-sensitized irradiation of f-butyl-2,5-dihydro-5,5-dimethyl-2-0X0-IH-pyrrole-l-carboxylate has been determined. Styryldicyanopyrazines undergo topochemical dimerisation when they are irradiated in the crystalline phase.  

The photocycloaddition of chloroprene to methyl 2,4-dioxopentanoate (4) has been reported. Only two de Mayo style products were obtained from this process and these were identified as the adducts (5) and (6) arising from the two paths of addition of the enol (7) to the diene. Precise kinetic data has been obtained for the photochemical dimerisation of the cyclopentanone derivative (8).  

A review has highlighted the photocycloaddition reactions of alkenes with aromatic esters and nitriles. Cycloadditions occur by a (3+2)-mode and provides a path to medium size ring systems. When the cinnamic acid derivative (9) is irradiated at 359 nm in ethanol with added Ti02 the product (10) is formed in 30% yield. Analogous products are formed from other straight chain alcohols such as (11) from propan-l-ol. Benzonitrile can be photochemically hydrated in the presence of oxophosphorus porphyrins.  

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Open-chained systems containing the disubstituted benzene subunit. . . 321... 

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems, due primarily to the prevention of extensive rotation about the C,—S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate and the racemization rate may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

Another common situation that can lead to second order spectra is an open chain system such as meso-l,2-difluoro-l,2-phenylethane whose magnetically nonequivalent spin system and resultant second order fluorine NMR spectrum (Fig. 2.7) can only be understood by examination of the contributing conformations about its fluorine bearing carbons.10... 

Let us now consider the conformations in some open chain system. 

Before illustrating the scope of this method for the synthesis of complex structures, there are examples in the literature where non-conjugated dienes are in open-chain systems but... 

As pointed out earlier, there is a growing interest in the control that can be exercised on the reactions of molecules constrained within the solid state. The previous focus on this dealt with rearrangements of dibenzobarrelenes, but control has also been demonstrated for open-chain systems. This has been shown by Demuth and his coworkers203 in a study... 

Methylenesulphones are more acidic than the simple esters, ketones and cyano compounds and are more reactive with haloalkanes [e.g. 48-57] to yield precursors for the synthesis of aldehydes [53], ketones [53], esters [54] and 1,4-diketones [55] (Scheme 6.4). The early extractive alkylation methods have been superseded by solidtliquid phase-transfer catalytic methods [e.g. 58] and, combined with microwave irradiation, the reaction times are reduced dramatically [59]. The reactions appear to be somewhat sensitive to steric hindrance, as the methylenesulphones tend to be unreactive towards secondary haloalkanes and it has been reported that iodomethylsulphones cannot be dialkylated [49], although mono- and di-chloromethylsulphones are alkylated with no difficulty [48, 60] and methylenesulphones react with dihaloalkanes to yield cycloalkyl sulphones (Table 6.5 and 6.6). When the ratio of dihaloalkane to methylene sulphone is greater than 0.5 1, open chain systems are produced [48, 49]. Vinyl sulphones are obtained from the base-catalysed elimination of the halogen acid from the products of the alkylation of halomethylenesulphones [48]. 

Method F (reaction with dihaloalkanes to give open-chain products) The dihaloalkane (0.01 mol) in PhH (5 ml) is added dropwise to the methylenesulphone (0.02 mol), TEBA-C1 (0.2 g, 0.8 mmol) in PhH and aqueous NaOH (50%, 15 ml). The mixture is stirred at 25-30°C for ca. 3 h and is then poured into H20 (150 ml) and extracted with CH2C12 (3 x 30 ml). The extracts are washed well with H20, dried (MgS04), and evaporated to yield the open-chain systems. 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

For an open chain system with (4rH- 2) 7T-electrons, the reverse is true and hence cyclization and ring rupture are in this case disrotatory. This description of the process shows why the conrotatory process is favoured in the cyclobutene isomerization. It does not rule out the reverse process where the conrotatory process is energetically very unfavourable. 

The simplest systems, and most closely related to the open chain systems discussed in the preceding section, are cyclic amidines, such as 2-iminopyrrolidine ([55], n = 3), 2-iminopiperidine ([55],n = 4) and their N-alkyl derivatives. Infrared spectra of their hydrochloride... 

Open-chain systems containing conjugated N and C basic centres are found in enamines [150] (sometimes called vinylogous amines) and the isoelectronic hydrazones [151]. 

In open-chain systems nmr indications of predominant O- (or C-) protonation are the restricted rotation around the Q-N bond and the slowness of NH-exchange. The nmr spectrum of j3-dimethylamino-acrolein in aqueous perchloric acid shows that the non-equivalence of the two N-methyl groups is retained (Kramer and Gompper, 1964). S-Benzylaminoacrolein is also mainly 0-protonated (Kramer, 1966). 

In the second group of ring-chain tautomeric interconversions, an open-chain system is transformed into a cyclic system through the intramolecular reversible addition of a functional group to a polar multiple bond lA IB 2A 2B 3A 3B and 4A 4B. The book (I) and this article deal with... 

IV. Polar Cycloadditions Involving Open-Chain Systems. ... 

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