Vinyl iminophosphorane

Another total synthesis used the rich chemistry of iminophosphoranes (95AHC159). The /3-(3-indolyl)vinyl iminophosphorane 354 underwent an aza-Wittig/electrocyclic ring closure reaction to give the carboline 355 which was hydrolyzed with lithium hydroxide (Scheme 106). A selective reduction, deprotection, decarboxylation and diazotation followed by ring closure gave Fascaplysine (353) (94TL8851). 

The absolute configuration of the structurally unique fungal metabolite mycosporins was determined in the laboratory of J.D. White by means of enantioselective total synthesis." In the endgame of the synthetic effort, the Staudinger reaction was used to elaborate the side chain. The cyclic vinyl azide was first converted to a stable vinyl iminophosphorane, which was subsequently reacted with benzyl glyoxylate to afford the corresponding Schiff base. Reduction of the imine was achieved with sodium cyanoborohydride. 

N-Vinylcarbodiimides 1732, useful building blocks for the synthesis of N-het-erocycles, are prepared from isocyanates 1730 and vinyl-iminophosphoranes 1731 in yields of about 70% [1268]. N-Vinylcarbodiimides 1732 react with tosyl isocyanate 1733 in a hetero-Diels-Alder reaction to form pyrimidines 1734 in 40-45% yield. 

In the reactions of iminophosphoranes with carbonyl and unsaturated compounds, both normal and abnormal aza-Wittig processes can be observed. Thus, the tandem aza-Wittig reaction of the iminophosphorane (6 with isocyanate or CS2 generated the 3,5-dthydro-6/f-imidazo[l,2-6]-l,2,4-triazol-6-ones in satisfactory yields, and the vinyl iminophosphorane (6 was transformed by normal aza-Wittig reactions to carbodiimides (as part of the synthetic route to 3,5-dihydro-6//-imidazo[l,2-i)]-l,2,4-triazol-6-ones). By contrast, a re-examination of the reaction of the iminophosphorane (67) with the aromatic isocyanate PhNCO showed that it gives an unexpected mixture of carbodiimides." In another example (Scheme 5), the... 

Reactivity studies on the iminophosphoranes have revealed that N-alkyl derivatives show a higher reactivity than A-aryl or A-vinyl derivatives. The order of reactivity of the heterocumulenes shown in Scheme 33 is given in Scheme 34 (21HCA861). 

Very few 2/f-triazole syntheses involve azides, apart from those (such as 2-acetyl derivatives) which involve prior formation and rearrangement of IjEf-triazoles. Some vinyl diazides react with triphenyl-phosphine in mild conditions to give iminophosphoranes of 2-amino-triazoles (Scheme 36). ... 

Vinyl acetate or 2-propenyl acetate, toluene, Cp 2Sm(THF)2, rt, 3 h, 88-99% yield. Other esters can also be prepared by this method. Iminophosphorane bases also serve as excellent transesterification catalysts with vinyl acetate (74-99% yield). ... 

In related work, it has been shown that 2-azido-3-vinyl-1,4-naphthoquinones also behave abnormally in their reactions with triphenylphosphine, undergoing reduction of the azido group and oxidation of the unsaturated carbon-carbon side chain, Phosphazides (143), having a linear structure, rather than iminophosphoranes, have been isolated from the reactions of tri-isopropylphosphine with r-alkylazides. Iminophosphoranes have also been isolated from the... 

Phosphoranylideneaminoquinones, e.g. (20), have been prepared in good to excellent yields by the reaction of phosphines with [2,l]benzisoxazole-4,7-quinones, e.g. (19)." A safer alternative to the vinyl azide-based synthesis of (N-vinylimino)phosphoranes (21) has been reported (Scheme 1)." N-(AlkyIthiomethyl)iminophosphoranes (23) have been synthesised from the benzotriazole-substituted iminophosphorane (22) and used without isolation to provide routes to N-alkylthiomethyl-imines, -amides, and -ureas. Tetraazolyl-substituted ylides (25) and (26) are the products of the reaction of vinylphosphonium salts (24) with excess sodium azide." The novel 1,2-1 -azaphosphete (28), which is potentially antiaromatic, has been prepared by the reaction of the azide... 

A-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 -I- 21-cycloaddition-cycloreversion sequence. " The presence of an alkyl substituent in position 3 of A-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed." Other Aza-Wittig reactions include the reaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia, useful carbodiimides for the selective synthesis of pyrimidones." Also the iminophosphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on temperature and aldehyde, trans imines or mixtures of trans and cis imines." " The... 

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