Room-temperature, endo-specific 1-aza-1,3-butadiene Diels-Alder reactions: acceleration of the LUMOdiene-controlled [4 + 2] cycloaddition reactions through noncomplementary aza diene substitution

Electron-deficient 1- and 2-azabuta-1,3-dienes: a comprehensive survey of their synthesis and reactivity

“…Boger and co-workers studied the 4p-participation of 1-sulfonyl-1-azabuta-1,3-dienes in regiospecific and endo-specific inverse electron demand hetero-Diels-Alder reactions, suitable for the diastereoselective preparation of substituted 1,2,3,4-tetrahydropyridines. 9, [96][97][98][99][100][101][102] The b,g-unsaturated N-sulfonylimines 158 and 159 proved to be accessible through clean, homolytic rearrangement of in situ generated oxime O-sulfinyl compounds 157 or through direct condensation of sulfonamides with a,b-unsaturated aldehydes 20 (Scheme 48). 100 Initial studies showed that both thermal and pressure promoted DA-cycloadditions of 1-phenylsulfonyl-1-azabuta-1,3-dienes with a broad range of electron-rich olefins cleanly provided the hetero-Diels-Alder cycloadducts.…”

“…These azadienes proved also to be sufficiently reactive toward intermolecular [4+2]-cycloaddition reactions with relatively unreactive olefins (styrene, p-methoxystyrene). 34,97 To investigate the influence of an electron-withdrawing group at the 4-position of the 1-sulfonyl-1-azabuta-1,3-dienes, 4-ethoxycarbonyl-1-methylsulfonyl-1-aza-1,3-diene 167a and 4-ethoxycarbonyl-1-phenylsulfonyl-1-aza-1,3-diene 167b were prepared. Intermolecular [4+2]-cycloaddition reactions of 167 with a broad range of dienophiles 168 showed that the noncomplementary C4 addition of an electron-withdrawing group to 167 substantially accelerated the rate of the DA-cycloaddition (Scheme 51, Table 2).…”

“…In addition, as a consequence of the boat transition state for the [4+2]-cycloaddition reaction, the lone pair on nitrogen and the s C-O bond of the dienophile lie trans periplanar to each other in the preferred endo-transition state suggesting a n/s* stabilization of the endo transition state. 9,34,96,97,102 To further extend the scope of DA-cycloaddition reactions of 1-sulfonyl-1-azabuta-1,3-dienes, intramolecular Diels-Alder reactions of these azadienes with unactivated dienophiles were studied in detail. In accordance with the observations made in the intermolecular cycloaddition reactions, a strong endo diastereoselectivity (420 : 1) was observed in the case of azadiene 171.…”

(E,E)-N-Phenylsulfonyl-4-phenyl-1-aza-1,3-butadiene

“…2a, 3 N-Benzenesulfonyl aldimines have been shown to be more reactive than N-benzenesulfonyl ketimines (eq 2), 1,2a,3 and the complementary addition of a C-3 or the noncomplementary addition of C-2 and C-4 electron-withdrawing substituents substantially accelerate the Diels-Alder reaction and maintain the expected cycloaddition regioselectivity and exceptionally high endo diastereoselectivity (eqs 3-5). 2a, [3][4][5] Synthetic procedures for preparation of highly substituted or highly functionalized pyridines based on the [4 + 2] cycloaddition reactions of N-sulfonyl-1-aza-1,3-butadienes have been developed and their applications in the total syntheses of fredericamycin A (2) (eq 6) 6 and streptonigrone (3) …”

Convenient Synthesis of N-Benzyl-1,4-dihydropyridines, Cyclohexenones, and Bicyclo[3.3.1]nonan-3-one Derivatives from 1-Aza-1,3-butadienes

“…1-Aza-1,3-butadiene derivatives (enimines) are reported to be useful intermediates in various synthetic procedures − and natural product syntheses. − Their use as electrophiles in 1,2- and 1,4-additions has been investigated, − and their utility as dienes in hetero Diels−Alder reactions continues to be the subject of considerable interest. − Recently, reports have appeared on the structure , and application , of metal-coordinated 1-aza-1,3-diene complexes, and an improved procedure for the preparation of cyclic enimines has been described . Our main interest regarding the 1-aza-1,3-butadienes has been to investigate whether enimines are suitable substrates in reactions that involve two consecutive steps, namely a Michael addition to the enimine with a subsequent ring-closure step …”