Ketenes, vinyl cycloaddition 1,3-dienes

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

Chiral dienes have proved to be less popular in asymmetric Diels-Alder reactions than their chiral dienophile counterparts. This is primarily a result of the problem of designing a molecule that incorporates a chiral moiety, such as the formation of a chiral isoprenyl ether or vinyl ketene acetal.187-190 In addition, diastereoselectivities often are not high,54 191-199 as illustrated by the cycloaddition of the chiral butadiene 5 with acrolein (Scheme 26.4). Improved stereoselection is observed through the use of double asymmetric induction, although this is a somewhat wasteful protocol.35,54 177 200... 

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... 

AAAcylimines, e.g., 4, are the most widely recognized and the most extensively investigated hetero-2-azadiene system capable of participation in Diels-Alder reactions, and comprehensive reviews have been published.7,71 In general, substituents X and Y are strongly electron-withdrawing groups and consequently the AAacylimines participate as electron-deficient partners in cycloaddition reactions with electron-rich dienophiles. Diels-Alder reactions of electron-deficient /V-acylimines with vinyl ethers, enamines, olefins, sulfenes, acetylenes, and the carbon-carbon or carbon-oxygen double bond of ketenes have been detailed.57 71 This 47t participation of electron-deficient AAacylimines does complement the ability of many simple AAacylimines to behave as 2ir dienophile components in Diels-Alder reactions with typical electron-rich dienes.6... 

The procedure for oxidation of the sulfide to the sulfone is based on that reported earlier by Bordwell and Pitt. The synthetic utility of phenyl vinyl sulfone and sulfoxide derives not only from their ability to serve as excellent Michael acceptors toward such reagents as enolate anions and organometallics, but also as moderately reactive dienophiles in Diels-Alder reactions. " The resulting adducts, in turn, can be chemically modified so that these electron-deficient olefins serve as useful synthons for acetylene,ethylene,terminal olefins,vinylsilanes, and ketene in [4 + 2] cycloadditions. Phenyl vinyl sulfone undergoes ready cycloaddition to Danishefsky s diene in the first step of a protocol for the regiospecific y-alkylation of 2-cyclohexenones. Furthermore, the ready lithiation of phenyl... 

Nitroethene and vinyl sulfoxides have also been employed as ketene equivalents. Nitroethene is an excellent dienophile and oxidation of the initial nitro-adduct gives the corresponding ketone. However, the thermal instability of nitroethene limits its appUcation to cycloadditions with reactive dienes. An attractive feature of vinyl sulfoxides as ketene equivalents is that they can be obtained in optically active form because of the chirality of the sulfoxide group, thus allowing enan-tioselective Diels-Alder reactions. Cycloaddition of p-tolyl vinyl sulfoxide with cyclopentadiene requires heat and gives a mixture of all four (two exo and two... 

Owing to the importance of p-lactam antibiotics, the [2+2]cycloadditions of ketenes to imines and isocyanates to olefins play a special role because both reactions lead to direct formation of a four-membered azetidinone-2 ring. The first one can be performed using a large variety of ketenes and ketene equivalents, whereas the second one leads to useful compounds only in the case of chlorosulfonyl isocyanate and vinyl acetates, vinyl silyl ethers, and dienes. One example of the addition of an isocyanate to a vinyl ether has been reported (Scheme 1). ... 

A detailed study of the cycloaddition of trifluoromethylethene has appeared as has work on a-siloxy-a -unsaturated ketones as dienophiles under Lewis-acid catalysis. Amongst the new dienophiles that have been reported are the two novel ketene equivalents methyl methoxypropiolate (129) and methyl phenyl-thiopropiolate (130). A series of 3-(acyloxy)but-3-en-2-ones (131) has been prepared and used as dienophiles in a route to anthracylinones, and, as part of a route to highly functionalized cyclohexenones, the vinyl sulphone (132) has been used in a Diels-Alder reaction with various dienes. ... 

The Influence of Dienophile Structure on Reactivity and Stereoselectivity in [4+2] Cycloadditions of Nitroalkenes Various dienophiles have been used successfully in nitroalkene [4 + 2] cycloadditions, such as vinyl ethers [77], enamines [78], ketene acetals [79], unactivated alkenes [58, 80, 81], 1,3-dienes [47, 82], allylic silanes [83], and allenes [84] (Scheme 16.10). Because of their high... 

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