Allylic alcohols spiroketalization

SCHEME 7 Reactivity hypothesis for allylic alcohol spiroketalization. 

Organotin hydrides can be used for the reductive transformation of a variety of functionalities. When an azide was treated with Bu3SnH in the presence of AIBN the azide group was converted to the desired amino group while spiroketal and allyl alcohol components remained intact (Scheme 12.114) [218]. 

For this chemistry, changing the allylic alcohol to a propargylic alcohol was necessary for spiroketal formation. We also felt that spiroketals could be formed from allylic alcohols and that they would provide alternative products, spiroketals with a vinyl substituent, which would offer access to different types of motifs for applications in synthesis. 

In addition, if a chiral secondary allylic alcohol is used, a stereospecific intramolecular oxypalladation and elimination can take place to give stereodeflned oxygen heterocycles such as tetrahy-drop)Tans (eq 11), 3,6-dihydro[2H]pyrans, dihydrofurans or spiroketals. ... 

The synthesis of the C1-C15 AB spiroketal 457 is described in Scheme 65. The synthesis of the C8-C15 fragment 453 started with allyl TMS 450, prepared by Evans asymmetric alkylation. Addition of 450 to aldehyde 451 with SnCl4 stereo-selectively afforded a-alcohol 452, which was converted into methyl ketone 453 through a sequence of four steps involving Mitsunobu inversion. The synthesis of... 

Upon treatment with a Pd(II) catalyst, keto allylic diols or derivatives deliver the corresponding spiroketal through oxypalladation on the activated double bond of the intermediate lactol (2013CEJ11613). Borrero and Apo-nick used this method to form the spiroketal core of acortatarin A from keto alcohol 16 that bears an aUylic methyl ether moiety. When alcohol 16 was treated with Pd(PhCN)2Cl2 in CH2CI2 at 0 °C, a spiroketahzation took place albeit with no diastereoselectivity as compounds 17 and 17 were obtained in a 1 1 ratio. After separation, spiroketal 17 was transformed into acortatarin A in five steps (Scheme 11) (2012JOC8410). 

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