Page 240 - Advanced Organic Chemistry Part B - Reactions & Synthesis
P. 240

212                                              O 2 S
                                              CH 3  CH 3
                                                        N
      CHAPTER 2                             CH 3
                                                       O
      Reactions of Carbon                       O  OTBS   CH
      Nucleophiles with                               21A   3 H 3 C CH 3
      Carbonyl Compounds                                              N
                                             2.5 eq TiCl 4  Ph
                                      PhCH  O
                                             3.0 eq Bu N     O       O  86%
                                                   3
                                                         OH     OTBS
                                                                       3:1 syn
                                                                  CH CH
                                       CH 3                         3  3 H C CH
                                               1.1 eq TiCl               3    3
                                CH                    4    CH                    N
                                  3                          3
                                         CH  O  1.1 eq i Pr NEt                 O    63%
                                                     2
                                                                         O  OTBS
                                                                    OH              3:2 syn
                                      CH 3                             CH CH
                                                                         3
                                               2.2 eq TiCl 4
                                                                            3 H 3 C  CH
                                               2.5 eq i Pr NEt                    3   N
                                                     2
                         (CH ) CO C(CH )  CH  O            (CH ) CO C(CH )
                                                                     2 3
                                                                 2
                                                             3 3
                                2
                                    2 3
                            3 3
                                                                                     O
                                                                       HO    O   OTBS
                                                                                    84% yield
                                                                                     ds > 20:1
                        2.22. The facial selectivity of the aldehydes 22A and 22B is dependent on both the
                             configuration at the  -center and the nature of the enolate as indicated by the
                             data below. Consider possible transition structures for these reactions and offer
                             a rationale for the observed facial selectivity.
                             TBDMSO  OR 4        O                 TBDMSO  OR 4 OH
                                        CH  O                                     O
                        TBDPSO                CH   CH(CH )
                                            +   3      3 2    TBDPSO                CH(CH )
                                                                                        3 2
                                      CH
                                CH 3 CH 3  3
                             22A                                      CH 3 CH CH 3
                                                                            3
                                   enolate     R 4    3,4-syn:anti ratio enolate  R 4  3,4-syn:anti ratio
                                    Li        MOM      97:3        Li     TES        66:34
                                    TiCl 4    MOM      84:16       TiCl 4  TES       60:40
                                    Bu B      MOM      63:38       Bu 2 B  TES       <5:95
                                      2
                                    (C H ) B  MOM                  (C 5 H ) B  TES   14:86
                                      5  11 2          52:48          11 2
                            TBDMSO   OR 4        O                           4
                                                                   TBDMSO
                                                                           OR
                                                                              OH  O
                                        CH  O
                       TBDPSO                 CH 3  CH(CH )
                                                                                         )
                                            +          3 2    TBDPSO                CH(CH 3 2
                          22B  CH 3 CH 3  CH 3                        CH     CH
                                                                        3 CH 3  3
                                   enolate     R 4  3,4-syn:anti ratio  enolate  R 4  3,4-syn:anti ratio
                                    Li        MOM      15:85      Li       TES       7:93
                                    TiCl 4    MOM      62:38      TiCl 4   TES       27:73
                                       B                          Bu B
                                    Bu 2      MOM      81:39        2      TES       45:55
                                    (C H ) B  MOM      50:50      (C H ) B  TES      52:48
                                      5
                                                                     11 2
                                                                   5
                                       11 2
                        2.23. Predict the stereochemical outcome of the following aldol addition reactions
                             involving chiral auxiliaries.
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