[-] · C23H24AsBrO · (2-Oxoisopentyl)triphenylarsonium Bromide · (MW 471.29)
(highly stereoselective alkenation reagents for the synthesis of (E)-b-enones; especially suitable for unstable aldehyde substrates in natural products synthesis)
Physical Data: mp 181-182 °C.
Solubility: insol ether; sol THF, MeCN, etc.
Preparative Methods: from Triphenylarsine and 1-bromo-3-methyl-2-butanone, which is obtained from 3-methyl-2-butanone and bromine in methanol.1
Handling, Storage, and Precautions: the toxicity of this reagent is unknown at present; use in a fume hood.
The synthesis of (E)-b-enones is usually accomplished by the Wittig or Horner-Emmons reaction. It has been shown that arsonium ylides with electron-withdrawing substituents in the alkylidene moiety are more reactive than the corresponding phosphorane2,3 and a facile approach to achieve the alkenation using arsonium salts has been disclosed.4,5
b-(E)-Enones are prepared in high yields with high stereoselectivity by reaction of (1) with aldehydes under PTC conditions (eq 1).6
Similarly, 2-(oxoheptyl)triphenylarsonium bromide (C25H28OAsBr, mp 156-159 °C) (4) reacts with aldehydes to give (5) (eq 2).6
The aldehyde (6) reacts with the arsonium salt (1) in acetonitrile to give the key intermediate (7) for the synthesis of brassinosteroid in 78% yield (eq 3).
The construction of the alkenyl side chain of PGF2a and its analogs has frequently been accomplished by alkenation of the Corey aldehyde (8) with phosphonic esters or phosphoranes.7 However, the reactions must be strictly controlled and elimination of the acetoxy group often occurs in large scale reactions. When the reaction of (9) and (4) is carried out in acetonitrile, the elimination product is formed predominantly. However, with the CH2Cl2/THF solvent system in which the salt (4) is more soluble, the elimination is completely prevented. Thus the unstable aldehyde (9) reacts smoothly with (4) to give PGF2a intermediate (10) in 63% overall yield from (8) (eq 4).6
The related arsonium salt (4-oxopent-2(E)-enyl)triphenylarsonium bromide (11) (mp 127-128 °C) is prepared from triphenylarsine and (E)-5-bromo-3-penten-2-one without solvent at 50 °C for 4 h in 43% yield (eq 5).
Aldehydes react with (11) in ether (trace H2O) and K2CO3 at 0-3 °C to give a mixture of (E,E) and (E,Z) isomers in 81% yield, with the former predominating. The latter can be easily isomerized to the (E,E) isomer when treated with catalytic amount of I2 under daylight.
A naturally occurring trail-breaking alarm pheromone of sea slug, navenone A, has been synthesized in 46% yield by a two-step alkenation sequence (eq 6).8 In contrast, an alternative seven-step sequence provided navenone A in 1.1% overall yield.9
Yao-Zeng Huang, Li-Lan Shi & Zhang-Lin Zhou
Shanghai Institute of Organic Chemistry, China