2-Propenyl group

Very recently, Cook and Danishefsky [24] reported an interesting regioselectivity of intramolecular Diels-Alder reactions reversed by the change in the dienophihc moieties from vinyl to allenyl group (Scheme 19). For R = 2-propenyl group, C is bonded to the methyl substituted carbon Cj of the cyclohexadienone ring. For R = 2,3-butadienyl, C is bonded to Cy... 

A number of examples of good stereoselectivity based on substituent control of reactant conformation have been identified. For example, 11 gives more than 96% stereoselectivity for the isomer in which the methyl and 2-propenyl groups are cisS5... 

KH/HMPT is also useful for selective 3-cleavage of tris(homoallylic) alcohols, as shown by a new synthesis of a-damascone (4). In general, l,l-dimethyl-2-pro-penyl and benzyl groups are cleaved more readily than a l-methyl-2-propenyl group,... 

This protocol is also effective for the cyclization of an allenylaldehyde, the synthetic utility of which has been demonstrated in the synthesis of (+)-testudinariol A (Scheme 16.89) [97]. Cyclization of an allenylaldehyde provides a ris-cyclopentanol bearing a 2-propenyl group at the C2 position. The reaction mechanism may be accounted for by coordination of Ni(0) with both the aldehyde and the proximal alle-nyl double bond in an eclipsed fashion with a pseudo-equatorial orientation of the side chain, oxidative cyclization to a metallacycle, followed by Me2Zn transmetalla-tion and reductive elimination. 

From a synthetic point of view the stereoselective construction of the tri-cyclodecane skeleton and the installation of the methyl group at C-5 and of the 2-propenyl group at C-7 pose significant challenges which were addressed differently in the five syntheses of kelsoene known to date. 

Another exception is the hydrolysis of isopropenyl glucopyranosides, where the a anomer is hydrolyzed 4 times faster than the (3 anomer, contrary to the expectation from the anomeric effect, that the (3 anomer should be hydrolyzed the faster, the behavior normally observed for a, (3-pairs of pyranosides. The reason for the anomalous behavior of the 2-propenyl glycosides is that cleavage occurs between 0-1 and the 2-propenyl group.216... 

This method has proven especially useful for the synthesis of a variety of natural products containing the prenyl (3-methyl-2-butenyl) or ferf-prenyl (1,1-dimethyl-2-propenyl) groups, as will be discussed in Sect. 4.2.3)... 

Methallyl is the 2-methyl-2-propenyl group. Wittig, G. Org. Synth., Coll Vol 4 1963, 964. 

It is also important to remember that configurational isomers are common and that these (Z)- and ( )-isomers have important geometric constraints, including such obvious features as the inability to introduce a strain-free (E) double bond into a ring with fewer than eight carbons. As was the case for alkanes, trivial names also intrude for substituted alkenes. Thus, the ethenyl oup (CH2=CH-) is commonly called vinyl and the 2-propenyl group (CH2=CH-CH2-) is called allyl. 

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