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A synthesis of 1,1-disubstituted 1,2,3,4-tetrahydroisoquinolines (6) was achieved in a highly efficient manner via Pictet-Spengler reaction of arylethylamines (1) and acyclic and cyclic ketones (2) using titanium (IV) isopropoxide and acetic-formic anhydride. The cyclization of the in situ formed acyliminium ion (4) to N-formyl 1,2,3,4-tetrahydroisoquinoline (5) was greatly facilitated by using trifluoroacetic acid as an additional reagent. The Pictet-Spengler reaction was carried out by one pot procedure, providing a convenient and effective method for preparing various 1,2,3,4-tetrahydroisoquinolines.

Four stereoisomers of 1,3-dimethyl-1,2,3,4-tetrahydroisoquinoline, an inducer of Parkinson-like syndrome, were synthesized by applying a new method of 1,2,3,4-tetrahydroisoquinoline (TIQ) synthesis utilizing the Pummerer reaction as a key step. The chiral centers at C-1 and C-3 were constructed by two routes starting from alaninol (3) and 1-phenylethylamine (4) as a chiral source. Enantiomerically pure 1,3-dimethyl-TIQs (1R,3S)-(2) [corrected], (1S,3R)-(ent-2) [corrected], (1S,3S)-(1) [corrected], and (1R,3R)-(ent-1) [corrected] were prepared in a stereochemically unambiguous manner from 3 in 11 steps (route I) and from 4 in 6 steps (route II). The conformations of tetrahydroisoquinoline ring in 1-methyl, 3-methyl, and 1,3-dimethyl-TIQs were discussed on the basis of their CD, 1H-NMR spectra, and steric energies.

The photocycloaddition reaction of 5-phenylfuran-2,3-dione (la) to 2-trimethylsilyloxybutadiene proceeded in a [2s+2s] manner with high regio- and stereo-selectivities to give a 2-oxabicyclo[3.2.0]heptane-3,4-dione 2 with 7-endo-OTMS-7-exo-vinyl stereochemistry. 5-Arylfuran-2,3-diones (1a-e) on similar photocycloaddition with l-phenyl-l-trimethylsilyloxyethylene gave the corresponding 7-aryl derivatives 3a-e with the same regio- and stereo-chemistries in good yields. This stereochemical result of O-endo selectivity is consistent with the prediction obtained from the stereo-selection rule proposed for the enone-olefin photocycloaddition reaction. The reaction provides an efficient method for the synthesis of poly-functionalized cyclobutane derivatives.

The thermal addition reaction of various aroylketenes (C) generated by the thermolysis of 5-aryldioxofurans (A) to 1-aryl-1-trimethylsilyloxyethylenes gave 1, 5-diarylpentane-1, 3, 5-triones (D) and 2, 6-diaryl-4H-pyran-4-ones (E). The introduction of electron withdrawing substituents in the aroylketene and of electron donating substituents facilitated the addition reaction. The observed substituent effects and the reaction mechanism are interpreted in terms of molecular orbital analyses.