Synthesis of (2R,3S,22R,23R)- and (2R,3S,22S,23S)-2,3,22,23-tetrahydroxy-B-homo-7a-oxa-5α-ergostan-7-ones, two new brassinolide analogues

(2R,3S,22R,23R)- and (2R,3S,22S,23S)-2,3,22,23-tetrahydroxy-B-homo-7a-oxa-5α-ergostan-7-ones (4a and 4b) were synthesized starting from the 5α-ergosta-2,7,22-triene (7). Osmilation of the 2,7,22 double bonds of 7 afforded the two hexols 6 which were rearranged by acidic treatment to (2R,3S,22R,23R)- and (2R,3S,22S,23S)-2,3,22,23-tetrahydroxy-5α-ergostan-7-ones (5a and 5b) whose structures were determined by chemical methods. Baeyer-Villiger oxidation of the 7-oxo group of 5a and 5b afforded 4a and 4b, respectively. The triene 7 was obtained in pure form by dehydrotosylation of the 3β-hydroxy-3′,5′-dioxo-4′-phenyl-5,8[1′,2′]- 1′,2′,4′-triazolidino-5α,8α-ergosta-6,22-diene 3-p-toluenesulfonate (8) and subsequent reduction with lithium dissolved in liquid ammonia of the obtained 3′,5′-dioxo-4′-phenyl-5,8[1′,2′]-1′,2′ ,4′-triazolidino-5α,8α-ergosta-2,6,22-triene (9).