Evolution of Routes for Asymmetric Total Synthesis of Cyclocitrinol Enabled by Type II [5+2] Cycloaddition†.

Main observation and conclusion: The asymmetric total synthesis of an unusual C25 steroid containing a unique bicyclo[4.4.1]undecene A/B ring system, resulting in the synthesis of cyclocitrinol (1) and its isomer Δ8,14‐cyclocitrinol (38), is reported. Initial attempts to construct the synthetically challenging bicyclo[4.4.1]undecene A/B ring system using a type II [5+2] cycloaddition showed that a chiral substituent at the allylic position of the alkene (C6, cyclocitrinol numbering) controlled the stereoselective outcome of the cycloaddition reaction. Late‐stage migration of the tetrasubstituted C8–C14 double bond in Δ8,14‐cyclocitrinol (38) to obtain cyclocitrinol (1) proved challenging, inspiring an alternative approach. The chiral β‐CH2OR group on the allylic substituent at C6 played a pivotal role both in controlling the diastereoselectivity of the type II [5+2] cycloaddition and retaining the C6 substituent under lithium–amine conditions. [ABSTRACT FROM AUTHOR]