2- Trimethylsilyloxybuta-1,3-diene

Trimethylsilyloxybuta- 1,3-dienes (Figure Si3.5) may be used as the 4% component in Diels-Alder reactions and in this capacity they have been incorporated into many major syntheses. 

It is of note that due to their inherent polarity, 2-trimethylsilyloxybuta-1,3-dienes undergo regiospecific Diels-Alder reactions. As exemplified in Equation Si3.11, hydrolysis of the trimethylsilyl enol ether present in the Diels-Alder product reveals a substituted cyclohexanone. 

All investigations on the use of Lewis acids in the hetero Diels-Alder reaction of carbonyl compounds clearly show that a careful adjustment of the Lewis acidity for a given system is neccessary. Especially with trimethylsilyloxybuta-dienes a Mukayama type aldol reaction can easily take place instead of the desired hetero Diels-Alder reaction. 

Recently, enhanced endo selectivity has been reported in the Diels-Alder reaction of fE -l-acetoxybuta-l,3-dienes with methyl fi-nitroacrylate The selectivity is compared with that of the reaction using l-methoxybuta-l,3-dienes and 1-trimethylsilyloxybuta-1,3-di-enes The degree of electron richness of a diene is an important consideration in endo eKO selectivity issues In particular, electron-rich dienes favor the formation of fixc-nitrocycload-ducts fEq 8 9 ... 

IPreparation of l-methoxy-3-trimethylsilyloxybuta-l, 3-diene (Danishefsky s Idiene) and Diels-Alder addition with maleic anhydride. 

IPreparation of 2-trimethylsilyloxybuta-l,3-diene and Diels-Alder addition Iwith diethyl fumarate. 

A Lewis acid-catalyzed cyclization of 2-aza-3-trimethylsilyloxybuta-l,3-diene has been reported in 2003 and the stereochemical differences with the uncatalyzed... 

Researchers in Chile have recently shown that treatment of the product obtained from condensation of 2-cyano-l,4-benzoquinone with (E)- -trimethylsilyloxybuta-1,3-diene with dilute hydrochloric acid results in smooth, high yielding conversion to the dihydrobenzofuran 1. 

Under Lewis acid catalysis, l-methoxy-3-trimethylsilyloxybuta-1,3-dienes react with aldehydes to give dihydropyrones (Equation Si3.14). 

The finding that 1-trimethylsilyloxybuta-1,3-dienes such as 23 react with acetals such as 24 not only in the expected 1.4-addition manner but also to give 5-alkoxy-a,p-unsaturated aldehydes in high yield and free from side products, established the silyloxydienes as important building blocks for carotenoid synthesis as shown in Scheme 5 for the synthesis of 25. 

A full paper on the preparation of 5-alkoxy-aj8-unsaturated aldehydes from 1-trimethylsilyloxybuta-1,3-diene and acetals has been published/ ... 

Diels-Alder Cycloadditions.—The range of functionalized diene systems, displaying regioselective behaviour in the Diels-Alder reaction, continues to expand. Notable new additions include l-phenylseleno-2-trimethylsilyloxy-4-methoxybuta-l,3-diene (82) and the isomers 1- and 2-methyl-l,3-bis(trimethylsilyloxy)buta-l,3-dienes (84) and (85). The diene (82) is readily derived from l-methoxy-3-trimethylsilyloxybuta-1,3-diene (81) and provides a direct route to the synthetically valuable 4-acylcyclohexa-2,5-dienones (83), whereas dienes (84) and (85) react with activated dienophiles (e.g. RCH=CHC02Et) in good yield, leading to keto-esters of the type (86) on hydrolysis. 

The Michael addition of the dienolate anion (90), readily generated from 1-trimethylsilyloxybuta-1,3-diene, to suitable acceptors provides an attractive alternative to the Diels-Alder reaction (Scheme 22). ... 

Cyclic Ketones.—Danishefsky has surveyed the Diels-Alder cycloaddition reactions of l-methoxy-3-trimethylsilyloxybuta-1,3-diene (76) (and its derivatives), which is available by enol silylation of the corresponding vinylogous ester [equation (41)]. These dienes react with a broad range of dienophiles to... 

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