Ring cleavage regiospecific

An elegant, general synthetic route to spirocarbocyclic systems (cf. Vol. 5, p. 55) involving regiospecific ring cleavage of appropriate tricyclic ketones [c/. (83)- (84)]... 

The photochemistry o oxaziridines has been independently investigated. Regiospecific photorearrangements of spiro-oxazirid-ines derived from 2-methylindan-1-one, 2-methyltetral-1-one and 1-methyltetral-2-one have been reported.The oxaziridines (65), for example, are converted on irradiation in ethanol into the lactams (66). A one-photon pathway resulting in ring cleavage and amide formation has also been observed in the gas-phase irradiation... 

A number of 1-substituted cyclopropanols are now available by nucleophilic attack on cyclopropanone, which may be generated in situ from its hemiacetals or other stable precursors. Although the cyclopropanols are obtained in these reactions without ring cleavage, nucleophilk decomposition of JV-alkyl-cyclopropanimine (274) gives a regiospecific C-1—C-3 cleavage (Scheme 41). ... 

An important stage in the synthesis has been reached. It was anticipated that cleavage of the trimethylsilyl enol ether in 18 using the procedure of Binkley and Heathcock18 would regiospecifically furnish the thermodynamic (more substituted) cyclopentanone enolate, a nucleophilic species that could then be alkylated with iodo-diyne 17. To secure what is to become the trans CD ring junction of the steroid nucleus, the diastereoisomer in which the vinyl and methyl substituents have a cis relationship must be formed. In the... 

Another example is the indirect acetonylation of a 1,2,3,4-tetrahydropyridine derivative at the (3-position of the enamide system [99]. The reaction involves a double alkylation to furnish an acetylcyclopropane which undergoes fragmentation on acid treatment. It is noted that polarity alternation and ring strain relief allow the facile and regiospecific C-C bond cleavage to proceed, the intervention of a cyclopropane intermediate also leads to disjoint system. This is a fundamental strategy [100] which has been frequently employed to gain access to compounds with disjoint functionalities. 

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