Group double bond formation

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Dehydrohalogenation of the 314 proceeded in excellent yield under the action of morpholine or piperidine at rt, during double bond formation between the C-l and C-2 atoms <2003CHE640>. The active methylene group of 3,4-dihydro-1 ///>//-[ 1,4 oxazino[3,4- quinazolin-6-onc 315 readily condensed with aromatic aldehydes at 160 °C in a melt to give the 1-benzylidenes, and coupled with aryldiazonium chlorides to give the arylhydrazono derivatives <1996BMC547>. 

The discovery that the iron-group elements can form bonds which have in part the character of multiple bonds by making use of the orbitals and electrons of the 3d subshell, whilq surprising, need not be greeted with skepticism the natural formula for a compound ECO is that with a double bond from R to C, and the existence of the metal carbonyls might well have been interpreted years ago as evidence for double-bond formation by metals. The double-bond structure for nickel tetracarbonyl (structure E) was in fact first proposed by Langmuir62 in 1921, on the basis of the electroneutrality principle, but at that time there was little support for the new idea. 

The problem of directed valence is treated from a group theory point of view. A method is developed by which the possibility of formation of covalent bonds in any spatial arrangement from a given electron configuration can be tested. The same method also determines the possibilities of double and triple bond formation. Previous results in the field of directed valence are extended to cover all possible configurations from two to eight s, p, or d electrons, and the possibilities of double bond formation in each case. A number of examples are discussed. 

In the previous papers no attempt has been made to discover all the possible stable electron groups which lead to directed valence bonds, nor have the possibilities of double bond formation been completely explored. In the present paper both of these deficiencies in the theory have been removed. 

The stereochemical implication for the ejection of a leaving group in the B-position of a carbonyl group which yields an a,B-unsaturated system has been treated elsewhere (see p. 233). It may be pointed out here that double bond formation through the opening of a cyclopropane ring should also take place following the same stereoelectronic principle. One example of such a reaction is the transformation of B,r-cyclopropyl-s-hydroxyketone 299 which is smoothly converted into the dienone 300 (85) under acid conditions. 

Electrolysis of 1,2-dibromofumaric and 1,2-dibromomaleic acid or the diesters produces a 90-100% yield of acetylenedicarboxylic acid or diester 302). For vie-dihalides with no radical or anion stabilizing group in the a position an E2B like elimination mechanism is strongly indicated, /.e., 2e-transfer and double bond formation in a synchronous process (Eq. (148)). 

Figure 10.9 Each class of sesquiterpenes (see Fig. 10.8) consists of many structural variants that arise from stereochemical positioning of methyl groups and sites of double bond formation. Depicted are the possible variants within the eremophilane class of sesquiterpenes, which are hypothesized to arise from an equally related class of sesquiterpene synthases. Only those structural derivatives that have been found in nature or synthesized are denoted by name. The remaining structures should, therefore, be considered novel.

Betaines, with 1,2-vinylene linking group double bond rotation barriers, 60, 226 formation, 60, 205, 2( -9. 213-4. 216 nmr spectra. 60, 224-6, 227 theoretical calculations, 60, 240-1 Bicyclo(2.2.0]hexa-2,5-dienes, see Dewar benzenes... 

Distinct functional groups with acidic hydrogens can also promote these transformations. For instance, benzoylnitromethane (208) or ethyl bromopyruvate (206) react with isoquinoline (6) and acetylenedicarboxylates via the same dipolar mechanism to generate a pyrrolo[2,l-a]isoquinoline scaffold. However, in these cases, after closure of the 5-membered ring, a double-bond formation via dehydrogenation or nitrous acid elimination yields the fully aromatic ring systems 207 and 209 (Scheme 28) [82, 183]. 

Various sets of functional groups have been devised for both types of decarboxylative carbon-carbon double-bond formation. Typical examples of a set of functional groups together with their electrolysis conditions are summarized in Table 8 [13-17,21,148,150-153]. 

The mass shifts of some fragment peaks in the MS-MS spectra of KR-60436 and its metabolites are summarized in Table 10.4 [14], From the interpretation of the data, four biotransformation processes can be recognized, each related to a specific site in the stracture (see the structirre in Table 10.4) (A) hydroxylation (mass shift of +16 Da), (B) double-bond formation (-2 Da), (C) loss of the hydroxyethyl side chain (-44 Da), and (D) demethylation (-14 Da). Combination of these processes takes place as well, e.g., in M2, where both double-bond formation and hydroxylation takes place. From the MS-MS data, it can be concluded that hydroxylation at (A) blocks the fragmentation of the pyridine ring, i.e., no loss of 84 Da is observed, and that double-bond formation at (B) blocks the formation of the fragment at m/z 150. The identification of the metabolites was as follows Ml and M2 contained a hydroxy-group in (A). M2, M4, M6, and M7 contained a donble bond in (B). M3 and M4 were demethylated at D. In M5 and M7, the hydroxyethyl group was lost [14]. 

---