NOPR Collection:

Synthesis of benzoxazinoid acetal glucosides naturally occurring in Gramineae

Sun, 01 Jun 1997 00:00:00 GMT

Title: Synthesis of benzoxazinoid acetal glucosides naturally occurring in Gramineae Authors: Sicker, Dieter; Hartenstein, Holger; Kluge, Michael Abstract: Benzoxazinoid acetal glucosides, a unique class of natural hemiacetal ethers from Gramineae and other species, are of interest as precursors of aglucones which exhibit high bioactivity as plant resistance factors against microbial diseases and insects, allelo chemicals and endogeneous ligands. This review gives a survey on synthetic methods which give rise to the hemiacetalic aglucones of the 2-hydroxy-2H-1 ,4-benzoxazin- 3(4H)-one skeleton, efficiently. As a result of investigations in the area of acetal glucosidation a double diastereoselective glucosidation method of racemic cyclic hemiacetals has been developed that affords an access to the acetal glucosides of natural configuration as well as to their enantiomers. Page(s): 461-475

Synthesis and properties of photochromic furofurans

Sun, 01 Jun 1997 00:00:00 GMT

Title: Synthesis and properties of photochromic furofurans Authors: Laatsch, Hartmut; Schmidt, Andreas Johann; Kral, Andreas; Heine, Niklas; Haucke, Gunter Abstract: The deep blue-violet bis-o-quinonemethide 3 cyclizes thermally in a [4_s+4_a]cycloaddition reaction, yielding the colourless [3,2- b]furofurane 1a. The latter is re-opened by irradiation with UV light back to the methide in its Z,Z- configuration 2a. By metallation of 1a, the highly photoreactive acetylene 8a is obtained which again forms a methide under irradiation, but now in its E,E-configuration 3. The properties of this unique new photochromic system and related furofurans and alkyneshave been investigated. Page(s): 476-484

Catalytic synthesis of 1,1,1,2-tetrafluoroethane from 1,1,1,2-tetrachloroethane - A mechanistic consideration

Sun, 01 Jun 1997 00:00:00 GMT

Title: Catalytic synthesis of 1,1,1,2-tetrafluoroethane from 1,1,1,2-tetrachloroethane - A mechanistic consideration Authors: Kemnitz, E; Niedersen, K-U; Kohne, A Abstract: 1,1,1,2-Tetrachloroethane and its fluorinated derivatives as well as trichloroethene are fluorinated by hydrogen fluoride in the presence of a pre-conditioned chromia catalyst. The reaction pathways are derived under different conditions. Fluorinated haloalkanes are formed both by dehydrochlorinationlhydrofluorination mechanism as well as chlorine/ fluorine exchange mechanism. Thus, beside fluorinated alkanes considerable amounts of haloolefins occur in the product mixture.A survey is given on the reaction pathway showing dependence on the reaction conditions. It is discussed with respect to calculated thermodynamic data. Kinetic and mechanistic investigations of the isomerisation reactions of 1.1.2.2-tetrafhioroethane on a CFC-conditioned chromia catalyst are also presented. The desired 1.1.1.2- tetrafluoroethane can be obtained from its symmetric isomer in the presence of a chromia catalyst conditioned exclusively with chlorine-free fluorocarbons. Mechanistic information is obtained by employing DCI which behaves similar to HF during the consecutive isomerisation reaction of 1.1.2.2-tetrafluoroethane. Thus, it is most probable that dehydrohalogenationlhydrohalogenation processes (elimination/addition mechanism) are mainly responsible for the formation of the haloolefins and halocarbons observed on chromia. Page(s): 485-493

Biomimetic radical polycyclizations of isoprenoid polyalkenes initiated by photoinduced electron transfer

Sun, 01 Jun 1997 00:00:00 GMT

Title: Biomimetic radical polycyclizations of isoprenoid polyalkenes initiated by photoinduced electron transfer Authors: Heinemann, Christoph; Warzecha, Klaus-D; Xing, Xuechao; Demuth, Martin Abstract: Isoprenoid polyalkene radicals, formed by anti- Markovnikov addition of a nucleophile to their parent radical cations, which are readily accessible via photoinduced electron transfer, undergo cascade cyclizations. The regioselectivity is efficiently controlled by the substitution pattern, i.e., the generally observed 6-endo-trig mode is replaced by 5-exo-trig, if electron-deficient double bonds (e.g. 1, 1-dicyanovinyl groups) are involved. Moreover, remarkably high asymmetric inductions have been achieved by the use of chiral spirocyclic dioxinones, derived from the chiral auxiliary (-)-menthone, notably remotely located from the initiationsite ofthe cyclizations.These asymmetricphotoinduced cyclizations constitute strong evidence of spontaneous coiling/folding of the terpenoid polyalkene chain and give ready access to the enantiomerically pure tricyclic terpenoids of complementary chiralities by means of the single chiral auxiliary (-)-menthone. Page(s): 494-497