The synthesis and some chemical reactions of a 5a-methyl 2H-naphtho[1,8-bc]furan

Abstract

A synthesis of the tricycle 19 has been completed in 14% overall yield from guaiacol. Several chemical transformations of this compound including its reduction to an octahydro naphtho[1,8-bc]furan and conversion to the hitherto unknown azuleno-[8,1-bc]furan ring system are described.

Total Synthesis of (+)-Xestoquinone Using an Asymmetric Palladium-Catalyzed Polyene Cyclization

“…Since the A ring of 51 could not be oxidized without destroying the molecule, we decided to develop a synthesis of 3-bromo-5,8-dimethoxy-2-naphthoyl chloride ( 38 ). If 38 could be successfully coupled with furan 29 and subsequently ring closed, then the oxidation of the para -oriented methoxy groups into a quinone would not be a problem since Harada oxidized a similar intermediate into xestoquinone …”

“…Initially, the asymmetric polyene cyclization was performed with Pd 2 (dba) 3 (( R )-(−)-BINAP) 4 , which provided 50 in 68% ee (78% yield). To determine if 50 had the correct absolute stereochemistry needed for (+)-xestoquinone ( 1 ), it was converted into xestoquinone by a catalytic hydrogenation to provide 52 followed by a ceric ammonium nitrate (CAN) oxidation (Scheme ). , Comparison of the CD spectrum (run in CH 3 CN) of our synthetic sample with a CD spectrum of (+)- 1 supplied to us by Professor Harada 6 indicated synthetic 1 (and thus 52 and 50 ) had the wrong absolute stereochemistry. Repeating the polyene cyclization on 44 using Pd 2 (dba) 3 (( S )-(+)-BINAP) 4 gave the pentacyclic product (+)- 50 with high yield and enantioselectivity (82% from 43 , 68% ee) .…”

“…Finally, (+)-xestoquinone ( 1 ) was prepared by a catalytic hydrogenation of (+)- 50 over 5% Pd/C (100%) followed by a CAN oxidation 6 of (+)- 52 . The 1 H NMR and CD spectra of synthetic (+)-xestoquinone ( 1 ) prepared using Pd 2 (dba) 3 (( S )-(+)-BINAP) 4 was identical with those provided by Prof. Harada …”

“… starting from the optically pure Wieland−Miescher ketone, and a formal total racemic synthesis, via a furan ring transfer reaction, has been reported by Kanematsu et al An examination of the structures of 1 and 2 suggested that the chiral quaternary center could readily be introduced by an asymmetric Heck or polyene cyclization including (+)-halenaquinone 6d8b however, it has not been applied to an enantioselective synthesis of a natural product.…”

Intramolecular Diels−Alder and Cope Reactions of o-Quinonoid Monoketals and Their Adducts: Efficient Syntheses of (±)-Xestoquinone and Heterocycles Related to Viridin

“…Although this work culminated in the synthesis of 2a (1.5% overall, 14 steps), it was not efficient enough to be realistically regarded as a gateway to the viridin group of natural products. Our initial synthetic efforts toward a tricycle like 3 were not successful, but their failure forced us to reexamine the problem and to look for a more direct route to this deceptively simple compound. Not only would success in this venture permit the development of a rapid synthesis of 2a and 2b , but it would also allow explorations to commence toward a first synthesis of viridin.…”