Fused furans, formation

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

This rearrangement of allylic acetals can also be used for furan annelations, in which the formation of the new tetrahydrofuran ring is coupled with ring enlargement of the starting ring.3 The same dr-fused bicyclic tetrahydrofuran is formed from either one of the cis- or mwr-allylic diols used as starting materials. 

The structural determination of those bicyclic compounds remained under discussion for some time, until Wickstrom et al.44 could (1) confirm the formation of fused bicyclic OZT-sugars and, more important (2) ascertain the furan forms for aldoses and the pyran form for D-fructose (Scheme 21). Those results were later confirmed by Jochims et al. through the first NMR analysis performed on OZTs.45... 

Ketoses should react under a similar scheme. Indeed they do but an important problem in the chemistry of ketoses consists on the lack of selectivity due to (1) the complexity of their tautomeric equilibria and (2) their tendency to form tertiary oxocarbenium ions under acidic conditions. Thus, mixtures of open-chain, cyclic and dehydrated products are frequently obtained.7 The discussion about OZT structures obtained from D-fructose as proposed by Zemplen, Wickstrom and more recently by Grouiller et al. continues today. In fact, the first authors claimed the fusion of OZT on a pyran form of D-fructose, while Grouiller suggested the formation of a mixture of fused OZTs with /1-pyran (major) and p-furan (minor) forms (Scheme 22).18... 

Although a limited range of Grignard reagents is available, the most widely used group is undoubtedly the lithio group introduced by direct lithiation (see Section 3.3.1.6.2). The ready formation of the lithio derivatives of pyrroles, furans and thiophenes and their benzo-fused derivatives has had a most important impact on the chemistry of these heterocyclic systems. Reaction of the... 

There are several reports of tetrahydroisoquinolines with a fused furan ring that could be argued (for the sake of the classification used in this collection) as an oxidative attack by the a-hydrogen of the 1-benzyl onto the 8-HO substituent, in a manner similar to the formation of a seven-membered ether ring seen in the cularines. It can also be seen as a similar oxidative attack from an a-hydroxy group (a commonly encountered benzyl substituent) on the 8-hydrogen position. The first of these two mechanisms (illustrated above) is used in this collection. 

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