Synthesis and Configurations of (-)-Furospongin-1 and (+)-Dihydrofuro­spongin-2

• Published in: European journal of organic chemistry Vol. 2016; no. 5; pp. 946 - 957
• Main Authors: Tan, Dong-Xing, Xu, Ze-Jun, Chen, Hui-Jun, Wu, Yikang, You, Jun
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.02.2016; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chiral pool; Configuration determination; Natural products; Oxygen heterocycles; Terpenes; Total synthesis
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201501489

The long‐known furanoterpenes furospongin‐1 and dihydrofurospongin‐2 were synthesized for the first time using a chiral‐pool‐based route in an effort to secure the previous configurational assignments. The key C‐11 stereogenic centre was taken from D‐mannose, and the C‐13 alkyl centre was installed exploiting the chirality of mannose. Due to deprotonation and/or enolization of the building blocks used, introduction of the furan moieties was problematic, and so some reactions had to be avoided. The trisubstituted alkene was most satisfactorily constructed using a Julia–Kocienski olefination in 1,2‐dimethoxyethane, with the best (E)/(Z) ratio achieved using a secondary sulfone. The synthetic samples not only provided the first unequivocal piece of evidence for the C‐13 configuration of both natural products, but also confirmed the absolute configuration at C‐11 of furospongin‐1. Despite repeated revisions over the years, the C‐13 absolute configuration of both furospongin‐1 and dihydrospongin‐2 remains questionable, as critical comparisons of optical rotations were made using data recorded in very different solvents. These doubts are now cleared using synthetic samples obtained by a chiral‐pool‐based route. The configuration of furospongin‐1 at C‐11 is also fully secured.