Alkyl silyl ketals

The type of atoms in the tether have little effect on the cycloaddition process. Both alkyl (98,125-127) and ether tethers (128-131) have been extensively investigated and provide similar levels of reactivity. In addition, thioethers (132), amines (133), silanes (134), and silyl ketals (135) are all compatible with the dipolar cycloaddition. 

Tertiary halides, acetates and methyl ethers, aryl-activated secondary halides, prenyl halides and acetates, acetals, ketals, thioacetals, a-chloroalkyl ethers, and, particularly, a-chloroalkyl phenyl sulrides have been widely used as alkylating agents. Such reagents may contain a variety of functional groups including alkenes, esters, halides, ketones, nitro groups and silyl ethers. 

After a-metalation, methoxyallene was alkylated with bromo silyl ether 47 to afford 48. A second metalation at the terminus of the allene was quenched sequentially with carbon dioxide and methyl iodide to give the corresponding allenic ester. Hydrolysis of the enol ether and equilibration gave solely the (E)-y-keto acrylate 49 in 30% overall yield. Ketalization then provided 16, which had previously been converted to pyrenophorin (9). ° ... 

Alkyl- and aryl-thiosilanes react with aldehydes or ketones to form either thio-ketals or O-silylated hemithioketals. With a/3-unsaturated ketones exclusive 1,4-addition can be achieved (Scheme 80). The procedures constitute an exceptionally mild method for carbonyl group protection. ... 

The diaIkeny1cyclopropano1 (403) is available from cyclo-propanone hemi-ketal (402) (Scheme 75). Thermal ring expansion gave the 3-a IkenyIcyc lopentanone silyl enol ether (404), which was regiospecifically alkylated to give, after desily lation, 11-deoxy-PGE methyl ester... 

The reaction may be of some preparative interest for obtaining alkylbenzonitriles and various a-functionalized alkylbenzonitriles starting from polynitriles (see Scheme 4.10 and Scheme 4.11) [56,57]. Donors that can be conveniently used as the precursors of the radicals include Jt donors, such as alkenes [58,59] and alkyl aromatics [60-63], heteroatom-centered donors, such as carboxylic acids [64] and ierf-butyl esters [65], ethers [66], ketals [67] (as well as cyclopropanone sUyl ketals) [68] and amines, organometallic donors such as silanes, silyl ethers, and silyl amines [69-71] as well as germanes, stannanes, and borates [72]. 

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