Page 239 - Advanced Organic Chemistry Part B - Reactions & Synthesis
P. 239

b.                                                                            211
                                                                                             PROBLEMS
                                                                OH
                                                            O
                          O     LDFA, THF then                       OPMB
                                –90°C    O   CH
                                                   OPMB      anti,anti
                     PMB = p -methoxybenzyl

                   c. The facial selectivity of 2-benzyloxy-3-pentanone toward typical alkyl,
                     alkenyl, and aryl aldehydes is reversed by a change of catalyst from TiCl to
                                                                                4
                     
 CH   CHO TiCl .
                                    3
                         3 2
                           OH                     O                             OH
                       O                                                    O
                               [(CH ) CHO]TiCl                  TiCl
                             R    3 2     3           + RCH  O     4              R
                                           PhCH O
                 PhCH O  CH 3                  2                      PhCH O  CH 3
                     2
                                                                          2
                     anti,syn                                             syn,syn
                   d. The boron enolates generated from ketones 20A and 20B give more than 95%
                     selectivity for the anti,anti diastereomer.

                                               H ) BCl
                              CH 3       1) (c-C 6 11 2           CH 3  OR
                      PhCH O               Et 3 N        PhCH O
                                                             2
                          2
                                     OR                                     CH 3
                                         2) CH CH CH  O
                                                 2
                                              3
                                O                                   O  OH
                         20A R = CH 3
                         20B R = CH 2 Ph
                   e.

                                  CH 3      O                     CH 3  CN   O
                      CH O                             CH O
                                                         3
                         3
                            CH 3     OTBDMS                  CH 3     OTBDMS

                   f.

                         O
                                                        O
                                                                 CH 3
                                CH   CH 3
                            CO 2  3  +     CH  O
                                                       CH
                          CH 3                            3    OH

               2.21. The camphor sultam derivative 21A was used in a synthesis of epothilone.
                    The stereoselectivity of the aldol addition was examined with several different
                    aldehydes. Discuss the factors that lead to the variable stereoselectivity in the
                    three cases shown.
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