Carbamates alkadienyl

Titanated 2,4-alkadienyl carbamates add to aldehydes also with y-regioselectivity16. 

In recent years, Hoppe s group has considerably extended the isomerization-addition methodology, especially for the highly regio- and stereoselective synthesis of 1,2-alkadienyl carbamates. It involves deprotonation of alkynyl carbamates, transme-talation into a titanium species and subsequent reaction with carbonyl compounds [26-30]. This group recently described the preparation of enantiomerically enriched 4-hydroxyallenyl carbamates 22 by sparteine-mediated lithiation of alkynyl carbamates 20 [29]. Impressive examples of these transformations are summarized in Scheme 8.8. 

An alternative synthesis of dihydrofurans was recently described by Hoppe and co-workers [84], The easily accessible 4-hydroxy-l,2-alkadienyl carbamates 106 were treated with Nal in the presence of NCS, which resulted in the formation of 3-iodo-2,5-dihydrofurans 107 (Scheme 8.28). Syn,syn-106 and syn,anti-106 gave the corresponding enantiopure 3-iodo compounds 107a,b in moderate to good yield. By treatment of syn,syn-106 with iodine in DMSO at 90 °C, the authors obtained the corresponding 3-iodofuran [84]. 

Table 4. 1,2-Alkadienyl Diisopropylcarbamates from Propynyl Carbamates via Ad-...

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