Page 177 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry
P. 177

P1: GOL Final Pages
 Encyclopedia of Physical Science and Technology  EN007F-314  July 6, 2001  16:59







              Heterocyclic Chemistry                                                                      333

                                                                                 m-CPBA
                                                                                                  O
                                                                                dichloromethane
                     X             X            X
                  5-exo-trig product       6-endo-trig product
                Ring expansion of aziridine to a tetrahydrooxazole is                NaOCl        O
                                                                               manganese salen
              accelerated by the release of angle strain in the former.
                                                                         Ph          complex  Ph
                     H            O              O
                     N
                         +  n-C 6 H 13    n-C 6 H 13
                                                                                                   O
                                  H               N                                t     i
                                                  H                              Bu OOH, Ti(O Pr) 4
                           a ring expansion of aziridine                     OH  (-)-diethyl tartrate  OH
                                                                     R 1                        R 1
                                                                           O  +  CH 2 N 2            O
                                                                     R 2                       R 2
              V. MAJOR CLASSES OF
                                                                                synthesis of epoxides
                HETEROCYCLIC COMPOUNDS
                                                                  The conversion of oxirane into thiirane proceeds via as
              A. Three- and Four-Membered Rings
                                                                S N ANRORC mechanism.
                1. Synthesis
                                                                                   N            _
              The saturated three-membered rings containing a nitro-                             N
                                                                                    C
              gen, oxygen, or sulfur atom are respectively referred to as  O  KSCN  O _  S
              aziridine, oxirane, and thiirane. They are usually readily                       O   S
              formed by a ring closure that is entropically favored. The  oxirane
              analogous formulation of four-membered rings (n = 2) is
                                                                               NCO
              less favored and, unsurprisingly, less common.                                              _
                                                                                                    + NCO
                                                                                                S
                                                                                    _  S
                                                                                            preparation of thiirane
                        (CH 2 ) n       (CH 2 ) n
                      HZ    CH 2 Br   Z    CH 2
                                                                  The formation of aziridine is illustrated by the following
                                 Na 2 S
                      Br      Br            thietane
                                        S                       sequence.
                       cyclizations that give small heterocyclic rings
                                                                               +
                                                                             H 3 N            H
                                                                                       NaOH   N
                Epoxides (oxiranes) are extremely useful synthetic in-               _  H 2 O
                                                                                    SO 3
              termediates (see Section V.A.2 below). Most double bonds
                                                                                   formation of aziridine
              can be epoxidized by reaction with m-chloroperoxy-
              benzoic acid (m-CPBA). Electron-deficient double bonds
                                                                  Important routes to four-membered rings containing
              are an exception, but can be epoxidized using alkaline
                                                                one (or more) heteroatoms are [2 + 2]cycloadditions, as
              hydrogen peroxide.
                                                                previously discussed. Ring expansion of cyclopropanone
                Some alkenes undergo enantioselective epoxidation
              upon treatment with hypochlorite in the presence of a chi-
              ral manganese catalyst (Jacobsen reaction). The epoxida-    O                          O
                                                                            HCHO         PhCH=NPh
              tion of allylic alcohols by means of an alkylhydroperoxide  O       C  C  O          NPh
              attached to a titanium alkoxide–tartrate system is known                         Ph
                                                                                               an azetidinone
              as the Sharpless asymmetric epoxidation and is one of
              the most general and widely used asymmetric processes
              known; high enantioselectivity is usually observed.  to azetidinones is also practical.
                Carbonyl compounds react with diazoalkanes with loss
              of nitrogen to give epoxides. Indeed, addition of a car-                   Cl
                                                                 O     HO  NHCH 2 COOH  HO  NHCH 2 COOR  O
              bene (R 2 C:) or a nitrene (RN:) (or a carbene or nitrene          t BuOCl        Ag +
              equivalent) to a double bond is another general route to                                NCH 2 COOR
              three-membered heterocycles.                                 azetidinones via ring expansion
   172   173   174   175   176   177   178   179   180   181   182