Vinylallenes synthesis

Van t Hoff t-factors 565 Vinylallenes rearrangement of 748 synthesis of 737 Vinyl carbonium ions 620 17a-Vinyl-17/f-hydroxysteroids, epimerization of 735 Vinyl sulphides, as alkyl sulphoxide reduction products 930, 932 Vinyl sulphones - see also Alkenyl... 

Okamura W. H., Curtin M. L. Pericyclization of Vinylallenes in Organic Synthesis on the Intramolecnlar Diels-Alder Reaction Synlett 1990 1 9... 

Based on the facile formation and reactivity of 323, and the retro Diels-Alder reaction of 325306,310 a simple procedure has been developed for the stereoselective synthesis of functionalized conjugated dienes as well as vinylallenes (see equation 119). 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Further examples of the use of the hDA reaction in dihydropyran synthesis include the formation of the fused pyrans 18 from vinylallenes 17 and aldehydes (Scheme 8) <00TL6781> and a trans-fused dihydropyran containing a phosphonate group 19 . A total synthesis of the 11-oxa steroid system is based on an intramolecular Diels-Alder reaction involving an orthoquinodimethane derived from a benzocyclobutene (Scheme 9) <00TL1767>. 

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

Due to the distance between the stereogenic center and the place of the nucleophilic attack, the enantioselective 1,5-substitution of chiral enyne acetates constitutes one of the rare cases of remote stereocontrol in organocopper chemistry. Moreover, the method is not limited to substrate 51, but can also be applied to the synthesis of enantiomerically enriched or pure vinylallenes 53-57 with variable substituent patterns (Scheme 2.20) [28]. 

Scheme 4.13 Asymmetric synthesis of vinylallenes by the remote stereocontrol.

The nucleophilic attack on an acceptor-substituted allene can also take place at the acceptor itself, especially in the case of carbonyl groups of aldehydes, ketones or esters. Allenic esters are reduced to the corresponding primary alcohols by means of diisobutylaluminum hydride [18] and the synthesis of a vinylallene (allenene) by Peterson olefination of an allenyl ketone has also been reported [172]. The nucleophilic attack of allenylboranes 189 on butadienals 188 was investigated intensively by Wang and co-workers (Scheme 7.31) [184, 203, 248, 249]. The stereochemistry of the obtained secondary alcohol 190 depends on the substitution pattern. Fortunately, the synthesis of the desired Z-configured hepta-l,2,4-trien-6-ynes 191 is possible both by syn-elimination with the help of potassium hydride and by anti-elimination induced by sulfuric acid. Analogous allylboranes instead of the allenes 189 can be reacted also with the aldehydes 188 [250]. 

Intramolecular [4 + 2]-cycloadditions of vinylallenes have been utilized in the synthesis of complex molecules including natural products such as compactin. Heating the vinylallene 208 at 140 °C gave hexahydronaphthalene 209. The crude mixture was immediately reduced with LiB(sBu)3H because of the instability of /3,y-unsatu-rated ketones [175]. 

Scheme 16.41 Synthesis of (vinylallene) rhodium (I II) complex of planar structure.

A diastereoselective synthesis of the racemic vinylallene 96 was described by Gordon and Tabacchi [96] in 1992 (Scheme 18.31). Here, the exocyclic allene was formed via anti-SN2 -substitution of the propargylic sulfmate 98 with lithium diisopropenyl-cyanocuprate, which, however, was also found to undergo side reactions such as reduction and direct attack of the nucleophile at the sulfmate. Deprotection of the substitution product 99 (which was obtained with only 35% yield) finally provided the desired target molecule 96. 

The use of vinylallenes as the diene component in Diels-Alder reactions is very common, thus resulting in their ubiquitous use in natural product synthesis. A vinylal-lene has even been proposed by Schreiber and Kiessling [10] as a biogenetic intermediate in the synthesis of the skeleton of esperamicin A (32 —> 33). Their synthetic approach to esperamicin A (34) was modeled after this biogenetic proposal in which a Type II intramolecular Diels-Alder cycloaddition was used to gain access to the highly unsaturated bicyclic core of 34 (Scheme 19.8) [10]. 

A well-designed synthesis of the bottom portion of (+)-compactin (44) was reported by Keck and Kachensky using a vinylallene as the diene in an intramolecular Diels-Alder reaction (Scheme 19.10) [12]. This was done at a time when there was very little literature precedent on the use of vinylallenes as dienes. Based on examination of molecular models, it was reasoned that the transition state for the... 

Transition metal-catalyzed carbocycUzation reactions of tethered diene, enyne, diyne, and vinylallene derivatives represent an important class of transformations in synthetic organic chemistry. This may be attributed to the abihty to significantly increase molecular complexity through the highly selective combination of acyclic components, thereby facilitating the synthesis of complex polycychc products. Recently, rhodium-catalyzed carbocyclization reactions have attracted significant attention due to their immense synthetic versatility and the unique selectivities observed over a range of different transformations. This chapter provides an account of recent developments in rhodium-catalyzed [4-1-2] and [4-i-2-t2] carbocyclization reactions. 

More recently, Murakami and co-workers have developed a highly regioselechve method for the synthesis of substituted benzenes, which employs vinylallene derivatives and monosubshtuted alkynes (Eq. 2) [8]. Simple olefins, such as ethylene, are also applicable as dieneophile components, albeit with decreased efficiency (Eq. 3). A mechanistic raho-nale was presented for the proposed catalyhc cycle, which nicely explains the observed regioselectivity (Scheme 12.1). The regiochemical outcome was attributed to steric repulsion between the highly hindered metal center and the alkynyl substituent (20 versus 18). 

The vinylallene 110, derived from dibenzobarrelene, has been used as a precursor for penta-1,2,3,4-tetraene. An improved synthesis of 110 started with the conjugated dibromocyclopropyl ketone. Treatment of the ketone with excess methyllithium gave the desired compound 110 after treatment with alumina. Presumably the allene alcohol is involved in this series of reactions. ... 

Milder thermal conditions4,8 21 -23 make isomerizations of vinylallenes by [1,5] hydrogen shifts highly attractive for the synthesis of thermally labile polyenes. This approach has been widely explored in the synthesis of compounds of biological interest such as vitamins A and D and their analogs8,24. 

---