Total Synthesis of Brevetoxin B. 1. First Generation Strategies and New Approaches to Oxepane Systems

Abstract

The fust generation strategies toward the total synthesis of brevetoxin B (1) are presented and the syntheses of the key intermediates 3, 4, 5, 67, 83, and 94-98 required for the projected construction are described. The earliest and most convergent strategy required the application of the hydroxy epoxide cyclization and the intramolecular conjugate addition as key reactions for the construction of the fused tetrahydropyran ring systems (4) [ABC], (7) [FG], and (8) [UK]. The oxocene ring (H) was formed via a Wittig reaction followed by a hydroxy dithioketal cyclization to produce the hexacyclic fragment [FGHIJK] (6 5). The 12-membered dithionolactone 18 was envisioned as the precursor of the dioxepane system of the molecule via a projected bridging reaction, to construct simultaneously both oxepane rings. However, the dithionation of dilactone 17 proved unsuccessful. In a subsequently evolved strategy, a new photolytic approach toward the dioxepane region was developed, starting from the acyclic dithiono progenitor 20 (20 23). Application of this reaction to the brevetoxin B skeleton afforded the desired oxepene (96 97), which after deprotection produced oxepanone 98. A specifically designed reductive hydroxy ketone cyclization (98 99) was then employed in an attempt to close the remaining ring [E], but, again, without success. The novel rearrangement of hydroxy ketone 87 to the pentacyclic system 89 was observed in a less elaborate skeleton. The scope and generality of these silicon-induced reductive cyclizations are also described.