Vinyl‐ und Ethinylborane als Dienophile in der Diels‐Alder‐Cycloaddition mit inversem Elektronenbedarf. Vinyl‐ and Ethynylboranes as Dienophiles in the Diels‐Alder‐Cycloaddition with Inverse Electron Demand

Abstract

Vinylborane des Types A e B reagieren als Dienophile gleichermaBen rnit elektronenreichen, elektronenarmen und nicht aktivierten Dienenl-'). Sie werden deswegen als "omniphil" bezeichnet7

Synthesis of Highly Substituted Pyridazines through Alkynyl Boronic Ester Cycloaddition Reactions

“…6 We envisaged that employing alkynyl boronic esters in this reaction would allow the rapid assembly of highly substituted pyridazine boronic esters with potential control of the regiochemistry around the heteroaromatic ring (Scheme ). Indeed, Seitz and Haenel gave three examples of cycloaddition reactions of ethynyl boronic esters with symmetrical tetrazines that provided the corresponding pyridazines in good yield 7. The authors did not explore the use of more heavily substituted alkynyl boronic esters or investigate the regiochemistry of this reaction.…”

Boron-Substituted Building Blocks in Diels-Alder and Other Cycloaddition Reactions

“…Later, Seitz reported on the conversion of similar alkynyl boronic esters, such as 130, as well as an aza-analogue, compound 131, in the Diels-Alder reaction with inverseelectron demand (Scheme 47). 74 In this transformation, the tetrazine derivatives 128 and 129 reacted under reflux in xylene with the alkynyl boron derivatives to give cycloadducts 132 and 133 in good yields (72-75%) in a Diels-Alder/retro-Diels-Alder reaction sequence.…”

Dimethyl 1,2,4,5-Tetrazine-3,6-dicarboxylate

“…30 A similar approach has been described more recently for the preparation of other, structurally related C-glycosides. A wide variety of activated and unactivated dienophiles have been employed in cycloaddition reactions with 1, including uncommon reagents like vinyl-and ethynylboranes, 33 and ethynyltributyltin. 34 When an electron-rich C=C double bond of a (condensed) heterocyclic ring system serves as the second component in such "inverse" Diels-Alder reactions, polycyclic compounds can be prepared, usually in a single step.…”