Stereoselective synthesis of highly substituted 1-isomorphans (1-azabicyclo[3.3.1]nonanes)

We describe the first enantioselective synthesis of highly functionalized 1-azabicyclo[3.3.1]nonanes (1-IM). The 1-IM scaffold is present in natural products and drugs and is an isomer of the morphan moiety. The proposed methodology is based on an organocatalytic Michael addition of N-protected piperidine ketoesters to nitroalkenes and an intramolecular nitro-Mannich reaction as key transformations. The 1-IMs feature 6 contiguous stereocenters, substituents at positions 2 and 4, and nitro, ester, and hydroxyl functional groups at positions 3, 5, and 6 respectively. The synthesis is straightforward, highly stereoselective (up to 98% ee, >99 : 1 d.r.), with overall yields of up to 83% and requires only two purification steps.

Total Synthesis of Kopsone

A total synthesis of kopsone was achieved, featuring stereoselective preparation of an acyclic aldehyde having a protected hydroxylamine moiety via Ireland-Claisen rearrangement and intramolecular cycloaddition of an eight-membered cyclic nitrone to form the 2-azabicyclo[3.3.1]nonane skeleton.

Total Synthesis of Haliclonin A

The total synthesis of haliclonin A was accomplished. Starting from 3,5-dimethoxybenzoic acid, a functionalized cyclohexanone fused to a 17-membered ring was prepared through a Birch reduction/alkylation sequence, ring-closing metathesis, intramolecular cyclopropanation, and stereoselective 1,4-addition of an organocopper reagent to an enone moiety. Reductive C-N bond formation via an N,O-acetal forged the 3-azabicyclo[3.3.1]nonane core. The allyl alcohol moiety was constructed by a sequence involving stereoselective α-selenylation of an aldehyde via an enamine, syn-elimination of a selenoxide, and allylation of the aldehyde with an allylboronate. Formation of the 15-membered ring containing a skipped diene was achieved by ring-closing metathesis, and final transformations led to the synthesis of haliclonin A.