The suitability of anion-accelerated oxy-Cope rearrangement as a probe to study pi-facial selectivity. An experimental study with (6-methyl-1-oxa-4-thiaspiro[4.5]dec-6-en-7-yl)(allyl)methanols

摘要

(6-Methyl-1-oxa-4-thiaspiro[4.5]dec-6-en-7-yl)(allyl)methanols were subjected to anion-accelerated oxy-Cope rearrangement in solvents such as THF, benzene and DME to assess the pi-facial selectivity caused by the acetal function. The diastereomeric pairs 17a and 17b (R=Ph) furnished the same two diastereomeric products 18a and 18b, differing only in the relative preponderance. The reaction of 17a turned from mostly syn to oxygen (18a:18b=1:4) in THF to moderately syn to sulfur (18a:18b=2.2:1) in DME. The reaction was completely nonselective in benzene. The effect of solvent on the reaction of 17b was even more interesting. The reaction turned from completely nonselective (18a:18b=1:1) in THF to highly syn to sulfur (18a:18b=5.4:1) in DME. The selectivity in benzene was somewhere in between (18a:18b=2.7:1). Likewise, the reaction of 17b (R=H) turned from moderately syn to oxygen in THF (18a:18b=1:2) to nearly nonselective in DME (18a:18b=1:1.1). The rotation around the bond between the methanol carbon and the adjacent ring-carbon is restricted to allow rearrangement only syn to sulfur in 17a and syn to oxygen in 17b. The considerable erosion in the observed diastereoselectivity is due to a radical and/or ionic retroaldol-recombination process. The radical pathway, however, is more prevalent than the ionic alternative. The recombination favors somewhat the addition of the allyl radical to the cogenerated 7-benzoyl-6-methyl-1-oxa-4-thiaspiro[4.5]dec-6-ene on the face syn to the sulfur. As a result of such dissociation and recombination, the oxy-Cope rearrangement does not appear suitable as a probe for the study of diastereofacial selectivity. [References: 53