Cyclobutanones through SNi' ring closure, a mechanistic study

Mechanistic studies on the intramolecular nucleophilic substitution with allylic rearrangement (SNi' reaction) and a new stereoselective access to substituted cyclobutanones are reported. 4,4-Dialkyl-5-oxohex-2E-en-1-yl methanesulfonates 4 were converted to 2,2-dialkyl-3-vinylcyclobutanones 6 by SNi' ring closure. The stereochemical analysis of the reaction was achieved through ring closure of (6S)-6-chloro-3,3-diethylhept-4E-en-2-one (S)-17, defined by the absolute configuration of C(6), leading to (3S)-2,2-diethyl-3-(prop-1E-en-1-yl)cyclobutanone (S)-(E)-18 and (3R)-2,2-diethyl-3-(prop-1Z-en-1-yl)cyclobutanone (R)-(Z)-18, in a ratio of 85:15, with almost complete transfer of chirality (>97%). The absolute configuration of (S)-17 was determined by X-ray diffraction analysis of the camphanoate derivative 16. The absolute configuration of the cyclobutanone products (S)-(E)-18 and (R)-(Z)-18 was determined by Raman optical activity spectroscopy. Comparison of the absolute configuration of (S)-17 and the resulting (E)- and (Z)-cyclobutanones 18 allowed the conclusion that the SNi' reaction proceeds with syn geometry relative to the leaving group.