Triafulvenes with enamines

The rather complex reactivity exhibited by cyclopropenones on interaction with enamines (see p. 74) is not re-found in the reactions of triafulvenes with enamines and ketene acetals. Instead of a (3 + 3) cycloaddition of enamine C=C—N sequence to the CI(2)/C3 bond of triafulvene (as represented by ylide 51 J) the addition of the enamine double bond to triafulvene C /C2 bond (operating with cyclopropenones only as a minor side-reaction) predominates in all reactions hitherto investigated. 

Dicyano-substituted triafulvenes react with enamines to produce exclusively the cross-conjugated dicyanomethylene compounds 519, whose formation can be rationalized by a methylene bicyclo(2,l,0)pentane intermediate 51879 296 Since cyclanone enamines 520 and other cyclic enamines 522 react analogously, this C-C-insertion 237) of the triafulvene ring skeleton into the enamine C=C bond represents a versatile ring expansion mode (C + C3), which makes accessible a series of unsaturated medium-ring compounds (521/523) that are otherwise difficult to synthesize. 

Diacyl triafulvenes yield with enamines the 6-amino-5,6-dihydro-6aH-cyclo-penta(b)furans 524 of structure type A/B, type B is only obtained from enamines... 

Ketene acetals show a pattern of product formation very similiar to enamines79 Diphenyl-4,4-diacetyl triafulvene is converted to diacetylmethyl cyclopentadiene 529 by S,N-acetals, whilst diphenyl-4,4-dicyano triafulvene undergoes C—C-inser-tion to S,N- and N,N-acetals, e.g. 530/531, resulting in cross-conjugated systems 533/534 by analogy with enamines. Cyclic S,N-acetals 532, however, yield exclusively the bicyclic fulvenes 535 due to additional loss of methyl mercaptan. 

Treatment of cyclopropenones with isonitriles [56] [57] and of triafulvenes with isonitriles [58] gives cyclobutenes. Formations of expanded rings are observed by the reactions of enamines with diphenylcyclopropenones [59] [60]. 

In 1967, Ciabattoni and Nathan [86] described a cycloaddition of triafulvene 315 with enamines (e.g., 341 and 344), followed by ring opening to yield the den-dralene products 343 and 346 (Scheme 7.75) (see Chapter 1). 

Eicher and coworkers [87] have reported on a reaction of triafulvene 347 with enamine 348 to afford the 6,6 -dihydro-5//-cyclopenta[h]furan 349 (Scheme 7.76). 

On the other hand, 2,3-diphenyl-4,4-dicyanotriafulvene (1) is stable and inert to strong acids such as hydrobromic acid and tetrafluoroboric acid. Chlorine, however, adds to this triafulvene at the exocyclic C — C double bond to give a dichloride which regenerates the triafulvene upon zinc reduction. Although the dicyanotriafulvene 1 is also inert to most dienes, it undergoes [2 + 2] cycloadditions with nucleophilic enamines followed by ring opening. 

As noted by Tsuge and coworkers [88], the reactions of cyclic enamines (e.g., 350) with triafulvene 315 yields the medium ring compounds 351 (Scheme 7.77). 

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