Syntheses from acyclic precursors

Heating a mixture of 2,5-dimethyl-3-hexyne-2,5-diol (298) with elemental sulfur or selenium provides a one-pot synthesis of 3,6-dimethylthieno[3,2-6]thiophene (299) or 3,6-dimethyl-selenolo[3,2-6]selenophene (300) respectively 94H(38)143 . 

In this section the expressions acyclic or cyclic precursors are used to indicate whether a pyran-ring closure does or does not occur, respectively, during synthesis. No importance is attributed to the fact that reactants may already contain a ring. 

This approach to 4//-pyrans consisting in cyclodehydration of the 1,5-dicarbonyl derivatives has been used in many examples. 

5-Dialdehydes 3 were cyclodehydrated to methyl-4//-pyrans 4 (R = Me, R = H R = H, R = Me), and the same method was successfully used for the preparation of 2,5-dideuterated 2 from 1,4,4-trideuterio-glutaric dialdehyde (860MS459). 

One of the oxo groups in a 1,5-dioxo starting compound may be a part of the heterocyclic ring as found (82KGS317) for example in the synthesis of pyrazolo pyrans 14 (R = Me, Ph R = Ph, 3-02NC6H4) according to Eq. (2). 

This commonly used approach is demonstrated considering the typical products, 2/f-pyrans or 4//-pyrans. Typical substituent patterns in the synthesized molecules are also indicated. 

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SCHMIDT Stereoselective Synthesis from Acyclic Precursors... 

Synthesis from Acyclic Precursors 10.08.11.1 Pyrazolo[4,3-c]pyridines... 

Sec. III.A] PYRANS, THIOPYRANS, AND SELENOPYRANS III. Synthesis from Acyclic Precursors... 

Among the available methods to synthesize fused-ring thietes and bi- or tricyclic thietanes (but not spiro-com-pounds), the most efficient method seems to be the synthesis from acyclic precursors with the same number of carbons. Photocyclization reactions lead to thietes and thietanes in satisfactory to high yields. Mono- and disaccharides bearing fused thietane moieties have been synthesized using this method. 

Ring synthesis from acyclic precursors No information is available on the formation of these ring systems from acyclic precursors. 

Synthesis from Acyclic Precursors Abiotic Synthesis 419... 

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