Fulvene enamines

The preparation of fulvene enamines by the reaction of sodium cyclopent-adienide with dimethyl sulfate complexes of amides or lactams has recently been reported by several investigators [77, 78] (Eq. 22). 

Fulvene-type enamines, which possess some nonbenzenoid aromatic character, have been synthesized by treating cyclopentadienylsodium with an amide-dimethyl sulfate eomplex (117aJ17b) or quaternary pyridinium salts (117c). One of the simplest ones produced is 6-(dimethylamino)fulvene (117a,117d). 

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. 

Attack on Unsaturated Carbon. The annual addition of phosphites to every variety of activated double bond continues. These include nitro-alkenes,9 a/S-unsaturated carboxylic acid derivatives,10 maleimides,11 fulvenes,12 and pyridinium salts.13 The reaction of diethyl phosphite with keten 0,N-, S,N, and Al,AT-acetals has been used to prepare the enamine phosphonates (19).14... 

In several synthetic studies, cyclopropane derivatives were used as synthones or building elements for ring enlargement steps, e.g. reaction of enamines with cyclopropenone [65], synthesis of 2,3-dihydro-l,4-diazepine by thermal isomerization of 1,2-diamino-cyclopropanes [32] [66], and preparation of 3-amino-fulvenes from methylencyclopropenes with alkynamines [67]. 

Dienamines and enamines have been used in intermolecular [6 + 4]100 and intramolecular [6 + 4]101, 102 and [6 + 2]102 cycloadditions to fulvenes, probably via a zwitterionic intermediate101. Example of a [6 + 4] cycloaddition is given in equation 41102. The initial product from reaction between 194 and 195 eliminates the amine to produce 196. 

Quaternary anilinium salts (240), obtained from reaction of (pyrrol-l-yl)methanol and the appropriate aniline with trioctylphosphine, react in polar solvents via the azonia-fulvene ion (241) with enamines through a Mannich-type reaction followed by cyclization to give pyrrolizidines 242 and 243124. The ratio between the pyrrolizidines formed varies considerably with ring size of the enamine employed (equation 50). Pyrrolizines have also been obtained by reaction of pyrrole with two equivalents of enamine125 or condensation of some ketones with L-sodium or L-ethyl prolinate126. 

Amide chlorides were obtained by the same method from enamines, > 72 fulvenes, barbituric acid derivativesor pyridopyrimidines and PI salts (e.g. 52 equation 33). 

Ester-substituted fulvenes of the type 67 (equation 12) were found to undergo ready cycloadditions to enamines. Nucleophilic addition of 68 to the electrophilic 6-position of the fulvene, and cyclization of the resulting zwitterionic intermediate 69 with loss of diethylamine resulted in the formation of the tricyclic fulvene 70 (equation 12). 

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively reviewed4,5,200 and include their 4tt heterodiene Diels-Alder reactions with olefins,201-204 vinyl ethers,205 enamines,206 selected dienes,207-209 dipheny-lketenimines,210 ketenes,209,210 fulvenes,211 and selected heterocycles including furan,207-209,212 benzofuran,209,212,215 indoles,213 azepines,214 and 1,2-diazepines.214 The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cycloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products.4,5 With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 4- 2] cycloadditions [Eq. (52)],203,204... 

Reduction of Enamines. The reduction of enamines with NaBHsCN under acidic conditions proceeds via an iminium ion, similar to that seen in the reduction of imines. Examples, such as the reduction of an enamine-fulvene to the corresponding amine target (eq 45) and the reduction of the enamino-butenolide to the expected lactone (eq 46), are abundant in the current literature. 

In addition to enamines, other reactive olefins, such as vinyl ethers (12,111) cyclopentadiene (19,130.131.133,134 fulvenes (130,132.133,175.276 ... 

With these reagents the presence of an amine substituent on the 6-carbon atom markedly facilitates bromination [246]. This is an example of the way in which 6-amino groups activate the cyclopenta-diene ring to electrophilic attack these fulvenes can be regarded as enamines. Another example is provided by the ready formylation... 

Other reactive olefins, such as enamines and vinyl ethers also participate as dipolarophiles in their reaction with thiocarbonyl 5-imides. Even fulvenes undergo [3+2] cycloaddition reactions. 

Analogously, fulvene n-systems can act as a triene unit for enamines (Scheme 6.46a) [103], 2-oxyallyl cations (Scheme 6.46b) [104], chromium alkenyl carbene complexes (Scheme 6.46c) [105], 3-oxidopyrylium (Scheme 6.46d) [106], 3-oxidipyridinium betaines (Scheme 6.46e) [107], dienolates (Scheme 6.46f) [108], azirines (Scheme 6.46g) [109], and azomethine ylides (Scheme 6.46h) [110]. 

Erden and coworkers [35] reported a [64-2] cycloadditions of fulvenes 147 with in sitM-generated acetone pyrrolidine enamine to give 1,2-dihydropentalenes 148... 

Recently, Hayashi and coworkers reported the first example of an organocatalytic intramolecular [6-I-2] cycloaddition reaction of fulvenes via enamine catalysis of secondary amine 20. The authors found that cycloaddition of fulvenes 21, which... 

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