Results are reported that indicate that the rearrangement of hydroxy epoxide 2a to dihydropyran 3 which occurs with retention of configuration at C-8 proceeds either via a carbocation intermediate or by two successive substitution reactions, both with inversion of configuration, (C) 1999 Elsevier Science Ltd. All rights reserved.
The 8,13-epoxylabdane derivative rac-19 was prepared in eight steps commencing from (E,E)-farnesol (8). Key reactions are the formation of 7 from drimenal (9b) and 6, Sharpless I epoxidation (7 --> 11), epoxy alcohol oxidation (11 --> 16), reductive epoxide cleavage (16 --> 15), peracid epoxidation (15 --> 18), and the trimethylsilyl triflate-mediated conversion of 18 into 19.