2,3-Wittig rearrangements, allylic/propargylic ethers

Sigmatropic Wittig rearrangement of macrocyclic allyl propargyl ethers to carbocycles 91TA1. 

Table 5 2,3-Wittig Rearrangement of Allyl Propargyl Ethers...

Scheme 6 Envelope transition states for 2,3-Wittig rearrangements of propargyl allyl ethers...

Table 7a 2,3-Wittig Rearrangement of Nonracemic Allylic Propargylic Ethers Derived fiom (5)-Lactaldehyde. OSiBu Me2...

Allyl propargyl ethers are also good substrates for the Wittig rearrangement. Deprotonation of the propargyl position is relatively facile. 

In order to introduce substituents to a macro-cyclic framework in a stereoselective fashion, a 12,31 Wittig rearrangement of the corresponding macroeyclie allyl propargyl ether is an attractive strategy. Bulyllilhium in hexane-THI- promoted... 

The [2,3] Wittig rearrangement is a useful reaction involving a [2,3] sigmatropic shift, and has been used to obtain homopropargylic alcohols from allylic propargylic ethers with high stereoselectivity. The reaction has been modified by Marshall to provide a useful route to optically active allenic alcohols. 

ACYCLIC r2.31-WITTIG REARRANGEMENT OF PROPARGYL-ALLYL ETHERS... 

Wittig rearrangement of a-allyloxycarboxylic acid dianions and allyl propargylic dianions (Section 3.11.3.3) might be expected to proceed analogously. In fact, the same high preference for ( )-pro-ducts is observed, but the diastereoselectivity is reversed (Table 17, entries 5 versus 6). A chelated bicyclo [3.3.0] transition state readily explains the anti selectivity of (Z)-allylic ethers [Scheme 12, compare (R) with (T)]. The basis for syn selectivity observed with ( )-allylic ethers (Q) versus (S) is less clear. 

Entry 1 was the first example, reported in 1970 [40], of a highly stereoselective [2,3]-Wittig rearrangement, but comparison with entry 5 shows that only the Z isomer is selective. Entries 2-4 illustrate substituted propargyl Z allyl ethers. 

---