6_Asymmetric_Allylation

Chemistry 215 Handouts 中的一部分

Asymmetric Isoprenylation of Aldehydes

Diastereoselective Allylboration of Chiral, -Substituted Aldehydes

(+)-Ipc2BH

+

CH3 CH3

THF–25°C, 6 h

(+)-Ipc2B

CH3 CH3

The diastereofacial selectivity of the B-allyldiisopinocampheylborane reagent typically overrides any facial preference of the aldehyde for nucleophilic attack.

Hydroboration of allenes is an efficient me

thod for preparing B-allyldiisopinocamphenylboranes H3C (+)-Ipc2B CH3 CH3 RCHO, Et2O–78°C, 12 h; NaOH, H2O2 OH R H3C CH3 H 3C

O allylboration H Et2O,–78°C 81% H 3C H 3C

OH+ H3C H3C

OH

R CH3 n-C4H9 CH2=CH (CH3)2C=CH

yield (%) 73 79 70 85

ee (%) 91 92 95 96

MATCHED: MISMATCHED:

(–)-Ipc2BCH2CH=CH2 (+)-Ipc2BCH2CH=CH2

96 5

::

4 95

(92% de) (90% de)

O H 3C OBz allylboration H Et2O,–78°C 80% H 3C

OH+ OBz H3C

OH

OBz

Brown, H. C.; Jadhav, P. K. Tetrahedron Lett. 1984, 25, 1215-1218. Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. J. Org. Chem. 1986, 51, 432-439. Methallylation of Aldehydes RCHO, Et2O–78°C, 12 h; NaOH, H2O2 yield (%) 56 54 56 55 57 ee (%) 90 90 91 90 92 R

MISMATCHED: MATCHED:

(–)-Ipc2BCH2CH=CH2 (+)-Ipc2BCH2CH=CH2

94 4

::

6 96

(88% de) (92% de)

CH3 (+)-Ipc2BOCH3+ Li

Et2O (+)-Ipc2B–78°C, 1 h

CH3

OH

CH3

Although the stereochemical outcome of the allylboration of aldehydes using B-allyldiisopinocampheylborane is typically reagent controlled, this selectivity may be challenged with certain substrates:

R CH3 n-C3H7 n-C4H9 t-C4H9 CH2=CH

O allylboration H3C Ph H Et2O,–78°C 72% MISMATCHED: MATCHED: (–)-Ipc2BCH2CH=CH2 (+)-Ipc2BCH2CH=CH2 H 3C

OH+ Ph H3C

OH

Ph

67 2

::

33 98

(34% de) (96% de)

The yields for methallylation of aldehydes are generally lower than in simple allylation reactions. Brown, H. C.; Jadhav, P. K.; Perumal, P. T. Tetrahedron Lett. 1984, 25, 5111-5114. Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. J. Org. Chem. 1986, 51, 432-439. Brown, H. C.; Bhat, K. S.; Randad, R. S. J. Org. Chem. 1987, 52, 319-320. Brown, H. C.; Bhat, K. S.; Randad, R. S. J. Org. Chem. 1989, 54, 1570-1576. M. Movassaghi