Carbon ring closures with

A systematic effort to correlate ease of ring closure with the stereoelectronic requirements of the transition state has been developed by Baldwin and co-workers. They classify ring closures with respect to three factors (a) ring size, (b) the hybridization of the carbon at the reaction site, and (c) the relationship (endocyclic or exocyclic) of the reacting bond to the forming ring. Certain types of ring closures are found to be favorable whereas others are unfavorable for stereoelectronic reasons. The relationships are summarized in Table 3.12. 

In a scheme intended to produce a more highly substituted oxazolidone, epichlorohydrin is condensed with morpholine in the presence of strong base to give the aminoepoxide, 27. Ring opening of the oxirane by means of hydrazine gives the hydroxy-hydrazine (28). Ring closure with diethyl carbonate leads to the substituted oxazolidone (29). Condensation with 18 affords furaltadone (30). ... 

By Ring-Closure (With Formation of Two Heteroatom—Carbon Bonds)... 

Hydrazine 835 underwent ring closure with one carbon cyclizing reagents to afford 837 and/or /V-acyl derivatives 836. The N-acyl derivatives underwent cyclization on treatment with phosphorus oxychloride. They were also prepared by the reaction of hydrazones 838 with thionyl chloride (84JHC1565). Compound 835 underwent ring closure with carbon disulfide and ethyl chloroformate to give 839 (Scheme 169). 

Moreover, compounds of type 7-127, which were obtained from 7-125 by reaction with Pb(OAc)4, can undergo a further domino process when treated with potassium carbonate in a mixture of water and methanol [57]. This includes saponification of the acetate moieties in 7-127 to provide 7-129 via the unstable cyclic hemiacetal 7-128 (Scheme 7.35). Retro-Claisen reaction and ring closure with the proposed intermediates 7-130 and 7-131 led to the bridged ring-system 7-132 as a mixture of di-astereomers with preference of the (3-isomer. 

Ring closure with formation of heterocyclic derivatives may occur when a nucleophilic function is present in the starting alkene and it is suitably placed for cyclization [30-43]. Both kind of the mechanistic pathways shown in path a of Scheme 3 and in Scheme 4 may operate, as exemplified by Schemes 5-6 (Y = O, NR). Clearly, only in the first case carbon monoxide is incorporated into the heterocyclic ring (cyclocarbonylation). 

Conant and Lutz is a two-step process involving initial di-bromination of the diketone followed by ring closure with zinc and sodium iodide. The overall yield of traws-l,2-dibenzoyl-cyclopropane is approximately 15%. The submitters have extended the described method to the preparation of other frans-1,2-diaroylcyclopropanes, namely acidic hydrogens on both a-carbons. A probable mechanism for this transformation is as follows ... 

An asymmetric carbon-transfer reaction was also performed by using 2-(/>-tolyl)sulfmylmethyltetrahydro-l,3-oxa-zine 143 as the chiral aldehyde equivalent in the Pictet-Spengler ring closure with tryptamine, but only moderate diastereoselectivity ( 40% de) was observed in favor of the (l/ )-tetrahydro-/3-carboline 165, and the enantiopure main product could be isolated only in low yield (Scbeme 26) <2001H(55)1937, 2004T9171>. 

An interesting one-carbon ring closure is found in the reaction of 94 (prepared from phenylcyanothioformamide and 1,3-diphenylpropenone) with either/)-chlorobenzoyl chloride or phenyl isocyanate to give pyrrolo[3,2- thiazoles 95 and 96, respectively (Scheme 18) <1999PSS(148)117>. However, attempted extension of this reaction sequence to include acetic anhydride did not afford a pyrrolo[3,2-i71thiazole product. In a novel approach, the same workers obtained additional examples of pyrrolo[3,2- thiazoles 99 in excellent yield as the only isolable product by reaction of A-(4-chloro and 4-methylphenyl)cyanothioformamide 97 with arylidene 98 the formation of the product was rationalized via 100-103 as detailed in Scheme 19 <2005PS(180)573>. 

Synthesis of Heterocycles by Ring Closures with Carbon Monoxide J. Falbe, Newer Methods Prep. Org. Chem., 1971, 6, 193-222. 

Schulte and coworkers (62AP801) have prepared a number of 3-propargyl-4-hydroxy-2-pyrone derivatives (210) by condensation of /3-dicarbonyl compounds (208) with propargyl-malonyl chloride (209). On heating (210) with zinc carbonate, ring closure took place to give the corresponding furo[3,2-c]pyrone derivatives (211 Scheme 40). 

When 2-(3-phenylprop-2-ynyl)cyclohexane-l,3-dione is heated with zinc carbonate, ring closure to a fused 4//-pyran occurs (62AP645). If the triple bond is terminal, cyclization leads to a furan derivative. Under the same conditions, the analogous acyclic diketones do not yield pyrans. 

An alternative approach to synthesis of this ring system involves insertion of CO into the brominated secondary base (43), by treatment with carbon monoxide, lead tetra-acetate, and triphenylphosphine in tributylamine, the product being the amide (44), reduction of which affords the amine.81 Govadine (42 R1 = R4 = OMe, R2 = R5 = OH, R3 = H) has been synthesized by the conventional ring-closure, with formaldehyde, of the diphenolic base (45), or of its dibenzyl ether, in acid solution.82 Tetrahydroberberines, together with N-benzyltetrahydroisoquinolines, have also been obtained by the electrolytic reduction of 3,4-dihydroisoquinolines of structure (46).83... 

Procedure V. Ring Closure with Sulfuric Acid.198 For the conversion of Y-arylbutyric acids into cyclic ketones, the finely powdered acid (1 part) is added gradually with stirring to a mixture of concentrated sulfuric acid (3 volumes) and water (1 volume). After one hour s heating on the water bath the colored solution is cooled, diluted with water by pouring onto ice, and extracted with ether. The extract is washed with water, which removes traces of colored sulfonation product, then with dilute aqueous ammonia, and dried over anhydrous potassium carbonate the solvent is removed, the residue distilled under reduced pressure, and the distillate crystallized from a suitable solvent. 

Synthesis of heterocycies, by ring closures with carbon monoxide ... 

Triphenyl-1,3-dithiole (233) (Rj = R2 = R3 = C6H5) has been converted into the corresponding salt by means of trityl perchlorate.24 On the other hand, the 4,5-dicyano derivative (236) [which was first prepared by Bahr et al.li8 from carbon disulfide and ammonium cyanide by spontaneous desulfurization of the salts of cyanodithio-formic acid (235), followed by ring closure with diiodomethane] is stable towards various oxidizing agents. The ease of oxidation of 233 to 234 is probably dependent on the same structural factors which govern the preparation of dioxolenium salts from 1,3-dioxolanes.149... 

Meyers approach to a-chiral lactones may be expanded to the 8-lactone series with ee values comparable to those achieved for lactones (equation 118). Mercuri- and similarly halo-lactonizations of 8,8-unsaturated carboxylic acid derivatives are an efficient way to achieve stereocontrolled heteroatom-carbon ring closure, e.g. of (333) to (334 equation 1IQ). ... 

Formation of a cyanoformamidine by treatment of a symmetrical diphenylthiourea with potassium cyanide in alcohol containing lead carbonate, reduction with ammonium sulfide, and ring closure with concentrated sulfuric acid to isatin-2-anil also formed smoothly by ring closure of the cyanoformamidine with aluminum chloride in benzene or carbon disulfide. 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Examples of ring closures with c-amino mercapto heterocyclic compounds are the reactions of carbon disulfide with o-amino mercapto pyridines and o-amino mercapto quinolines, which respectively yield dihydrothiazolo pyridine-2-thiones and dihydrothiazolo qumohne-2-thiones. A summary of these compounds are 7 ... 

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