“…The synthesis of stable 2 H -pyrans is an ongoing challenge in organic synthesis . Since 2 H -pyrans undergo a reversible electrocyclic ring-opening to 1-oxatrienes, classical strategies toward monocyclic 2 H -pyrans generally afford an equilibrating mixture . Typically, the substrate-dependent equilibrium is dominated by 1-oxatrienes, 3d,g while the 2 H -pyran form is favored only in a few cases due to increased steric interactions. , Although there are numerous applications of 1-oxatrienes, , the synthetic utility of monocyclic 2 H -pyrans is somewhat limited. , …”
mentioning
confidence: 99%
“…We have also investigated further transformations of the 2 H -pyran products using known chemistry (Scheme ). For example, hydrogenation ( 3i → 5 ),3h dihydroxylation ( 3i → 6 ), and nucleophilic addition ( 3i → 7 ) have afforded the anticipated products in modest to good yields. 3 Synthetic Utility of 2 H -Pyrans …”
[reaction: see text] The application of easily accessed propargyl vinyl ethers for the synthesis of monocyclic 2H-pyrans was achieved. Under the reaction conditions, highly substituted heterocycles were obtained in moderate to excellent yields. The one-pot sequence proceeds via a Ag(I)-catalyzed propargyl-Claisen rearrangement, followed by a base-catalyzed isomerization, and 6pi-oxaelectrocyclization, leading to the formation of stable 2H-pyrans.
“…The synthesis of stable 2 H -pyrans is an ongoing challenge in organic synthesis . Since 2 H -pyrans undergo a reversible electrocyclic ring-opening to 1-oxatrienes, classical strategies toward monocyclic 2 H -pyrans generally afford an equilibrating mixture . Typically, the substrate-dependent equilibrium is dominated by 1-oxatrienes, 3d,g while the 2 H -pyran form is favored only in a few cases due to increased steric interactions. , Although there are numerous applications of 1-oxatrienes, , the synthetic utility of monocyclic 2 H -pyrans is somewhat limited. , …”
mentioning
confidence: 99%
“…We have also investigated further transformations of the 2 H -pyran products using known chemistry (Scheme ). For example, hydrogenation ( 3i → 5 ),3h dihydroxylation ( 3i → 6 ), and nucleophilic addition ( 3i → 7 ) have afforded the anticipated products in modest to good yields. 3 Synthetic Utility of 2 H -Pyrans …”
[reaction: see text] The application of easily accessed propargyl vinyl ethers for the synthesis of monocyclic 2H-pyrans was achieved. Under the reaction conditions, highly substituted heterocycles were obtained in moderate to excellent yields. The one-pot sequence proceeds via a Ag(I)-catalyzed propargyl-Claisen rearrangement, followed by a base-catalyzed isomerization, and 6pi-oxaelectrocyclization, leading to the formation of stable 2H-pyrans.
“…We report now the synthesis of dienic alcohols (3)-( 6), (ll), and (12), and y,S-(7), P,y-( 8)- (10), and a$,-unsaturated ketones (13), ( 14) by reaction of 2H-pyrans (1) -in equilibrium with their isomeric dienones (2) with organometallic compounds as shown in Scheme 1.…”
Section: Ring Opening Of 2h-pyrans Promoted By Organometallic Reagent...mentioning
“…It has been shown that 1,2-pyrans exist mainly in the oxodiene form and that substituents capable of enhancing the resonance in this isomer also displace the equilibrium in its favor (2)(3)(4)(5). The formation of a-pyrones was thought at first to proceed through the pyrilium salt because of the ease with which 5,5-dimethoxy-2-n-decylpenta-2,4-dienal (1) was converted to such a compound and since the intermediate 1,2-pyrans (orthoesters) were expected to be stable in basic media.…”
ALAIN B E L A N G E R~~~ PAUL BRASSARD. Can. J. Chem. 53,201 (1975). A simple one-step synthesis of a-pyrones and 3-chloro-a-pyrones from p-functionalized a,p-enones and ketene acetals has been devised. The method has also been adapted to the preparation of some 4-methoxy-a-pyrones. Finally the mechanism of the reaction has been investigated. ALAIN BELANGER et PAUL BRASSARD. Can. J. Chem. 53,201 (1975).Une synthkse simple d'a-pyrones et de chloro-3 a-pyrones en une ttape B partir d'a,p-&nones p-fonctionnalistes a e t t rtaliste. La mtthode s'adapte aussi B la prtparation de quelques mtthoxy-4 a-pyrones. Enfin le mCcanisme de la rtaction a Cte CtudiC.In a companion paper (1) we described a new synthesis of a-pyrones. This method involved the cycloaddition of chloroketene acetals to a,p-unsaturated carbonyl compounds followed by a dehydrohalogenation which yields a-pyrones. It was then assumed that placing the leaving-group on the enone should facilitate the elimination step, prevent the formation of by-products, and simplify the overall process. This modification uses the more readily accessible ketene dialkyl acetals and produces the a-pyrones directly in moderate to good yields (30-77%) (Scheme 1; R , = OAc, OBz, or C1; R, and R, = H, alkyl, or aryl). P-Functionalized enals and P-alkylenones are either unreactive or give only exchange reactions.The great stability of a 2,2,4-trimethoxy-3,4-dihydro-1,Zpyran suggested that the cycloaddition of chloroketene acetals to (3-methoxy enones and a subsequent dehydrohalogenation would provide a new synthesis of 4-methoxy-apyrones. The 3-chloro-4-methoxy-3,4-dihydro-1,2-pyrans are formed by heating the substrates at about 150" (35-80%) and converted to the expected a-pyrones with yields of 32-90%. (Scheme 2; R, = OMe; R, = H, R, = aryl or styryl; and R, = alkyl, R, = aryl).
Tile Reaction of 0-Functionalized Enones withKetene Acetals (Scheme 1) With P-Acetoxy and P-Benzoyloxy Enones The P-acetoxy enones 3a, 4a, 5a, and 7a react readily with ketene dimethyl acetal (I.) when heated at about 95" for 3-8 h. Examination of the 'Revision received August 2, 1974. n.m.r. spectra of the crude product usually reveals the presence of a mixture of the expected 4-acetoxy-2,2-dimethoxy-3,4-dihydro-1,2-pyran and elimination product, the 55-dimethoxypentadienone. The magnitude of the coupling constant between the protons in the 2 and 3 positions of the latter (-15 Hz) indicates that the tautomer has the trans-configuration (P-acetoxyacrylophenone reacts in 48 h at room temperature and gives only the relatively unstable dihydropyran).Since all attempts to separate the dihydropyran from the dienone were unsuccessful, the mixture was then heated to 120-130" when a vigorous reaction ensued. The a-pyrone could then be isolated directly by chromatography o n Florisil. Most variations of the experimental procedure such as the use of an inert solvent (xylene) or the addition of acetic acid to the reaction mixture produced no appreciable effect; no a-pyrone is obtained when the heating is carri...
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