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               332                                                                                Heterocyclic Chemistry


                 A general means of deoxygenating ROH into RH in-
               volves conversion of the hydroxy group into a thiocar-     S
               bonylimidazolide followed by reductive cleavage using      +  _                  S
               tributyltin hydride in the presence of the radical initiator  COOEt                  COOEt
               azobisisobutyronitrile (AIBN).
                                                                       m-CPBA                NaH
                          O
                              NHCOOEt              NHCOOEt                       O   O
                    N  N      O       Bu 3 SnH      O                              S         C 2 Cl 6
                          O
                         EtOOCN H     AIBN    EtOOCN H                                 COOEt
                               OR                   OR
                                                                                    - SO 2
                                                                         O   O
                 Benzotriazole ia an inexpensive and stable heterocycle    S                           COOEt
               that is soluble in a variety of common solvents. It is easily
                                                                               COOEt
               introduced into a molecule and can act as a satisfactory
               leaving group. Such properties enable its use in a wide
               variety of synthetic transformations, including displace-
               ments to form unsymmetrical secondary aliphatic amines.
                                                                 IV. SYNTHESIS OF HETEROCYCLES

                            N
                                      R 3 MgX  R 3
                              N   +              NHR 2           The synthesis of heterocyclic rings has much in common
                             N                R 1
                                NHR 2                            with carbocyclic rings. Ring formation can be via cycload-
                             R 1
                                                                 dition, ring closure, ring expansion or contraction, ring-
                                                                 atom interchange (ANRORC reactions), or insertion of
                 When borane becomes bonded to certain chiral oxaz-  reactive intermediates into double bonds.
               aborolidines, hydride transfer to ketones takes place, giv-
               ing mainly oneenantiomerofthe alcohol. Thelarger group  a  c  hν  a  d      b  a  f      b  a  f
               of the ketone points away from the heterocyclic ring, min-  b  +  d  b  c   c  +  e      c  e
                                                                                             d            d
               imizing steric hindrance.
                                                                      [2 + 2] cycloaddition   [4 + 2] cycloaddition
                                                                                +
                                      Ph
                                      H  Ph                                   _  b           b
                 O                                    H  OH
                          BH 3 .THF                                            a  c        a   c
               Ph     Cl    Ph       + N -  O  Me    Ph     Cl                 d  e         d  e
                            H  Ph      B
                                      B -  + O  Cl                              1,3-dipolar cycloaddition
                              O      H  H
                            N         H   Ph
                             B
                              CH 3
                                                                   The Woodward–Hoffmann rules predict the regio- and
                                                                 stereochemistry of substituents in rings formed by cy-
                 The ring opening of certain modified carbohydrates by
                                                                 cloaddition reactions. In practice, a number of cycload-
               mercury(II) acetate proceeds with subsequent cyclization
                                                                 dition reactions are rarely or never observed, and these
               (Ferrier rearrangement) to give a substituted cyclohex-
                                                                 are forbidden by these important rules. The extremely
               anone that can be reduced to an inositol derivative. Inos-
                                                                 useful Diels–Alder reaction is a [4 + 2]cycloaddition and
               itol phosphates are key messenger molecules in cellular
                                                                 was originally applied to carbocycles (atoms a–f = C), but
               signaling.
                                                                 it has been adapted to the synthesis of heterocycles.
                         OMe                   OH
                                                                                  Ph               Ph
                     RO                    RO     OCOCH 3                       N        ∆       N
                           O      i Hg(OCOCH 3 ) 2                            +
                                    ii NaCl                                     O                O
                              OCOCH 3
                     RO                    RO     O
                         OR                    OR                            synthesis of a dihydro-1,2-oxazine
                 Sulfur heterocycles readily form sulfur ylides that can  Ring-closure reactions follow well-established pat-
               undergo electrocyclic ring expansion to give a medium-  terns, which are summarized in Baldwin’s rules. For ex-
               size ring. The sulfur can then be used to link two carbon  ample, the ring closure of the terminal radical below leads
               atoms with extrusion of sulfur dioxide to form a cyclic  totheso-called5-Exo-Trigproductratherthanthe6-Endo-
               alkene (Ramberg–B¨acklund reaction).              Trig product.
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