Synthese und Reaktionen hochgespannter 2,3‐verbrückter 2H‐Azirine

Abstract

Trotz extremer Ringspannung und Reaktivität lassen sich die NMR‐spektroskopisch beobachtbaren und aus den Aziden 1 leicht zugänglichen Heterocyclen 2 in neuartigen Additions‐ und Cycloadditionsreaktionen stereoselektiv umsetzen.

The Chemistry of Vinyl, Allenyl, and Ethynyl Azides

“…38 The addition step is interpreted as a polar process via an iodonium ion to explain the electronically controlled regiochemistry leading to 33 and subsequently to the inner vinyl azide 34 but not to the terminal isomer 1 -azidohex -1 -ene. 40 When cycloalkenes with larger rings were used for the Hassner reaction including an ionic addition of iodine azide, the synthesis of vinyl azides was successful as shown by the sequence 47 → 48 → 49 . The stereochemistry of the addition products 36 shows that an anti addition has taken place, and the confi guration of the fi nal product 37 is a result of an anti elimination.…”

“…25c Small amounts of dihydropyrazine 175 , its tautomer, or the corresponding aromatic heterocycle were obtained on thermolysis of 172 in protic solvents or chromatography of this starting material. 40 But, a rapid subsequent reaction led to the formation of the dimers 186a -c , which were quickly oxidized to the corresponding aromatic pyrazines in the case of R = H. This dimerization can be catalyzed by traces of moisture. 124 This method is connected with the names of Hemetsberger and Knittel 63,67b and can be transferred to the synthesis of azaindoles, 125 fused indoles, 126 and fused pyrrol derivatives.…”

“…26b,63,123a However, heating of 171 (R 1 = aryl) in aprotic solvents, for example, xylene at 140 ° C, is more important and has been applied very frequently to prepare indoles of type 176 via intermediate 172 . 40 The slow reaction of simple 2 H -azirines with diazo compounds to afford allylic azides has been known for more than forty years. 127 Quite recently, reactions like 171 (R 1 = Ph) → 176 have been conducted in a rhodium(II) -catalyzed way, which allows to decrease the necessary temperature into a range of 25 -60 ° C. 128 The transformation of 171 into isoquinolines is also possible if one or both ortho positions of the aryl group R 1 are substituted appropriately.…”

“…127 Quite recently, reactions like 171 (R 1 = Ph) → 176 have been conducted in a rhodium(II) -catalyzed way, which allows to decrease the necessary temperature into a range of 25 -60 ° C. 128 The transformation of 171 into isoquinolines is also possible if one or both ortho positions of the aryl group R 1 are substituted appropriately. 40 Whereas alkyl -or aryl -substituted azirines without an electron -withdrawing group do not react with cyclopentadiene, 113b the corresponding Diels -Alder reactions of 185a and 185b were successful under mild conditions. 64f As opposed to this, the synthesis of similar pyrroles 178b from azides 177b was performed under mild conditions with the help of small amounts of zinc iodide.…”

“…129 When the pyrroles 178a were prepared by thermal cyclization of the precursors 177a in boiling xylene, the corresponding 2 H -azirines should be discussed as intermediates because the reaction conditions proved to be typical for the conversion of vinyl azides into these three -membered heterocycles. 40 Of the four imaginable diastereomeric [4 + 2] cycloadducts ( exoexo ; endoexo ; exoendo; endoendo ), the exoendo stereoisomers 189 were isolated exclusively. Several mechanisms were presented to explain the catalyzed formation of the fi ve -membered products.…”