Ynamines reactions with carbonyl compounds

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

By analogy with the formation of dihydropyrans from unsaturated carbonyl compounds and alkenes (see Section 2.24.2.7.l(i)), the synthesis of 4//-pyrans from the [4 + 23-cycloaddition of unsaturated carbonyl compounds and alkynes would seem to offer some potential. Such a reaction has indeed proved of value, but examples are largely restricted to the use of ynamines as the dienophile (76BSF987). 

The methods for generating acyl ketenes (Scheme 7-V) and their subsequent in situ participation in [4 + 2] cycloadditions with a wide range of hetero- or olefinic and acetylenic dienophiles (Scheme 7-VI), including acyl ketenes,185 186,197 carbonyl compounds, 86-188 nitriles,1874,189,191 isocyanates and isothiocyanates,1864,190,191 ketenes,191 imines,1864,1874,191,192 carbo-diimides,l87c 190,191,193 ynamines,194 ketene acetals,1864,195 enol ethers,1864,191,196 and V-sulfinylamines197 have been extensively reviewed.5,9,12 Two reports have detailed the 4-n- participation of allenic ketones in [4 + 2] cycloaddition reactions [Eq. (51)].198,199... 

Early extensive accounts of the 4v participation of a,/)-unsaturated carbonyl compounds in [4 + 2] cycloadditions detailed their reactions with electron-deficient dienophiles including a,/3-unsaturated nitriles, aldehydes, and ketones simple unactivated olefins including allylic alcohols and electron-rich dienophiles including enol ethers, enamines, vinyl carbamates, and vinyl ureas.23-25 31-33 Subsequent efforts have recognized the preferential participation of simple a,/3-unsaturated carbonyl compounds (a,/3-unsaturated aldehydes > ketones > esters) in inverse electron demand [4 + 2] cycloadditions and have further explored their [4 + 2]-cycloaddition reactions with enol ethers,34-48 acetylenic ethers,48 49 ke-tene acetals,36-50 enamines,4151-60-66 ynamines,61-63 ketene aminals,66 and selected simple olefins64-65 (Scheme 7-1). Additional examples may be found in Table 7-1. 

Fluoroalkenes have also been condensed with ynamines and fluoroallenes with thiols, and two groups have reported the reactions of ynamines with P-unsaturated carbonyl compounds. Here, the initial product may undergo a doublebond shift to give the conjugated P-amino-aP-unsaturated carbonyl derivative. An example of the synthetic utility of this reaction is shown in Scheme 22. 

SCHEME 26.52. Reaction of ynamines with various carbonyl compounds. 

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