Allylic Chromium Reagents

Allylic Chromium Reagents 12-Asymmetric Induction fAnti/Syn Control)... 

A related reaction has been described by Kitagawa and coworkers in which (126) exclusively provides (127) upon treatment with CrCb-LAJH (equation 51). The relative stereochemical result is qualitatively identical to that reported by Still and Mobilio. Cyclodecadienol (127) was subsequently transformed into ( )-costunolide. A third example of a related chromium-initiated cyclization was reported by Takeshita and coworkers in a synthesis of cycloaranosine (115). Serial treatment of (128) with MsCl/pyridine and CrCb-LAH provid (129) stereospecificaUy in 95% yield (equation 52). Another example of internal addition of an allylic chromium reagent to an aldehyde has been reported by Oshima and coworkers and is outlined in Section 1.6.3.6. ... 

Allylic acetals can be treated with CrCh to obtain the allylic chromium reagent, see K. Takai, K. Nitta and K. Utimoto, Tetrahedron Lett., 1988, 29, 5263. 

Figure 10-7. Proposed mechanism for the formation and addition of allylic chromium reagents.

The addition of the 2-butenylchromium reagents to the a-amino aldehydes give predominantly the iyn isomer 245 independently of the nature of the group bonded to the nitrogen (Table 10-27) [146b]. Ohta has recently examined the addition of allyl chromium reagents to Garner s aldehyde [147]. Poor to moderate diastereoselectivity is observed in these reactions. 

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