Evolution of natural product scaffolds by acyl- and arylnitroso hetero-diels-alder reactions: new chemistry on piperine.

Piperine, a natural product containing a conjugated diene, was reacted with polymer-supported acyl- and arylnitroso dienophiles. The reactions with arylnitroso dienophiles were also carried out in solution. The oxazine rings formed by the corresponding hetero-Diels-Alder reactions were further transformed and novel acyclic as well as heterocyclic derivatives including pyrroles and quinoxalinones were prepared.

New abietane diterpenoids from the mangrove Avicennia marina.

Phytochemical investigations of the twigs of Avicennia marina yielded three new abietane diterpenoids 11-hydroxy-8,11,13-abietatriene 12- O-beta-xylopyranoside ( 1), and a pair of inseparable epimers 6 Halpha-11,12,16-trihydroxy-6,7-secoabieta-8,11,13-triene-6,7-dial 11,6-hemiacetal ( 2) and 6 Hbeta-11,12,16-trihydroxy-6,7-secoabieta-8,11,13-triene-6,7-dial 11,6-hemiacetal ( 3), as well as the new lignan (7' S,8' R)-4,4',9'-trihydroxy-3,3',5,5'-tetramethoxy-7,8-dehydro-9-al-2,7'-cyclolignan ( 5), together with 6,11,12,16-tetrahydroxy-5,8,11,13-abitetetraen-7-one ( 4), lyoniresinol ( 6), lyoniresinol 9'- O-beta- D-glucopyranoside ( 7), and diacetylmartynoside ( 8). Structure elucidation of the new compounds was accomplished by analysis of their spectroscopic data. Compounds 2 - 4 showed moderate cytotoxic and antimicrobial activities.

Solid-supported nitroso hetero Diels-Alder reactions. 1. Acylnitroso dienophiles: scope and limitations.

Polymer-supported acylnitroso dienophiles were prepared and used in hetero Diels-Alder (HDA) reactions with a variety of dienes. The transient acylnitroso dienophiles were prepared in situ from immobilized hydroxamates, which were attached to solid supports via several linkers each cleavable by different cleavage reagents, and served for the synthesis of both N-unsubstituted and N-derivatized HDA adducts. Model compounds were used to (i) optimize reaction conditions for solid-supported HDA reactions, (ii) evaluate the outcome of the reactions with various dienes, (iii) compare relative reactivities of dienes, and (iv) assess the stability of HDA adducts toward cleavage conditions typically used in solid-phase syntheses.

Solid-supported nitroso hetero-Diels-Alder reactions. 3. Acid-mediated transformation of cycloadducts by scission of the oxazine C-O bonds.

Polymer-supported dihydro[1,2]oxazine derivatives were prepared by acyl- and arylnitroso hetero-Diels-Alder reactions and exposed to strong (trifluoroacetic) acid during cleavage from resin-bound linkers. Cycloadducts prepared from cyclic dienes containing electron-donating substituents at the C6 oxazine carbon promoted formation of carbocations by cleavage of the C-O bond. The carbocations were quenched by nucleophilic reagents including triethylsilane, water, and alcohols and provided access to novel derivatives of N-alkyl hydroxamates.

Solid-supported nitroso hetero diels-alder reactions. 2. Arylnitroso dienophiles: scope and limitations.

Immobilized arylnitroso dienophiles were prepared and used in hetero Diels-Alder (HDA) reactions with a variety of dienes. Polymer-supported arylnitroso species were prepared on several linkers cleavable by different cleavage reagents and used for (i) optimization of reaction conditions for HDA reactions, (ii) evaluation of the reaction outcome with various dienes, (iii) comparison of relative reactivities of dienes, and (iv) assessment of the stability of HDA adducts toward cleavage conditions typically used in solid-phase preparation (TFA). The outcome of the HDA reactions has been evaluated for a set of 19 dienes, and the relative reactivities of dienes that yielded the expected HDA adducts were compared.