Studies in Sulfur Heterocycles. Part 16: [1] [2] Synthesis of [1]Benzothieno[3,2-b]pyrans via Tandem Reactions from 2,3-Dihydrobenzo[b]thiophene-3(2H)ones

Abstract

Sequential treatment of N,N-diethyl-2-methylsulfanyl aryl amides with LDA and p-anisaldehyde, 1-naphthaldehyde and cinnamaldehyde afforded the corresponding thioaurones. Heating the thioaurones derived from cinnamaldehyde, above 200 °C resulted in electrocyclic ring closure and sigmatropic shift in tandem to give substituted [1]benzothieno[3,2-b]pyran. Treatment of N,N-diethyl-2-methylsulfanyl-5-methoxy benzamide and crotonaldehyde gave 4-methyl-8-methoxy[1]benzothieno[3,2-b]pyran via conjugate nucleophilic addition and ring closure in one pot. Possible mechanistic pathways are discussed.

thioaurones - electrocyclic ring closure - sigmatropic shift - benzothiopyran

References

1 Part 15, see: Ghosh SC. De A. J. Chem. Soc., Perkin Trans. 1  1999,  2639 

2

Abstracted in part from the PhD thesis of S. K submitted to Jadavpur University (2003).

3 Mukherjee C. De A. Synlett  2002,  325 

4

Mukherjee, C.; Kamila, S.; De, A. Tetrahedron 2003 (in press).

5 Cabiddu MG. Cabiddu S. Cabiddu E. de Montis S. Fattuoni C. Melis S. Usai M. Synthesis  2002,  875 

6a Perold GW. van Ligen PFA. Chem. Ber.  1959,  92:  253 

6b Guha SK. Chatterjea JN. Mishra AK. Chem. Ber.  1961,  94:  3297 

6c Cagniant P. Kirsch G. Cagniant D. C. R. Acad. Sci. Ser. C  1972,  74 

6d Hofman H. Westernacher H. Haberstroh HJ. Chem. Ber.  1973,  106:  349 

6e Wadsworth DH. Detty MR. J. Org. Chem.  1980,  45:  4611 

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7

Representative Experimental Procedure for the Preparation of the Thioaurones:
N,N-Diethyl-2-methylsulfanyl aryl amides were treated with LDA at 0 °C under argon and after a brief interval, the aldehyde was added by syringe. The temperature was kept at 0 °C for 1 h under inert atmosphere. Usual acidic work up after this period afforded the thioaurones in very good yields.
Compound 1: Yellowish fluffy solid, yield 83%, mp 152-153 °C. IR (KBr): 1635 cm^-1. ¹ H NMR (300 MHz, CDCl₃): δ = 7.93 (d, 1 H, J = 15.4 Hz), 7.67-7.64 (m, 2 H), 7.61 (s, 1 H), 7.45-7.41(m, 3 H), 7.38 (dd, 1 H, J = 7.9, 8.1 Hz), 7.10 (d, 1 H, J = 15.4 Hz), 6.97 (dd, 1 H, J = 7.9 Hz), 4.02 (s, 3 H). ¹³ C NMR (75 MHz, CDCl₃): δ = 186.4, 165.2, 154.8, 144.6, 134.8, 133.0, 132.6, 131.3, 129.4-129.1, 126.6, 122.9, 116.4, 109.5, 53.6. Anal. Calcd for C₁₈H₁₄O₂S: C, 73.40; H, 4.76. Found: C, 73.48; H, 4.74.
Compound 2: Yellowish solid, yield 74%, mp 146-147 °C. IR (KBr): 1639.4 cm^-1. ¹ H NMR (300 MHz, CDCl₃): δ = 7.80 (d, 1 H, J = 15.5 Hz), 7.59-7.55 (m, 2 H), 7.52 (s, 1 H), 7.37-7.35 (m, 3 H), 7.30 (d, 1 H, J = 2.4 Hz), 7.12 (dd, 1 H, J = 2.4, 8.8 Hz), 6.95 (d, 1 H, J = 15.5 Hz). ¹³ C NMR (75 MHz, CDCl₃): δ = 186.3, 164.4, 158.1, 144.6, 134.8, 132.7, 131.8, 131.3, 129.4-129.1, 124.5,122.8, 122.1, 56.0. Anal. Calcd for C₁₈H₁₄O₂S: C, 73.40; H, 4.76. Found: C, 73.10; H, 4.60.
Compound 6: Colourless needle shaped crystal, yield 78%, mp 194-195 °C. ¹ H NMR (300 MHz, CDCl₃): δ = 7.55-7.42 (m, 6 H), 7.35 (dd, 1 H, J = 7.8, 8.5 Hz), 6.93 (dd, 1 H, J = 7.8 Hz), 5.70 (dd, 1 H, J = 3.3, 3.4 Hz), 4.00 (s, 3 H), 3.22-2.88 (m, 2 H). ¹³ C NMR (75 MHz, CDCl₃): δ = 186.8, 161.5, 155.1, 138.4, 132.0, 130.7, 129.4, 129.3, 126.7, 126.6, 116.1, 115.8, 109.1, 83.2, 56.2, 44.4. Anal. Calcd for C₁₈H₁₄O₂S: C, 73.40; H, 4.76. Found: C, 73.50; H, 4.80.
Compound 7: Colourless solid, yield 65%, mp 207 °C. ¹ H NMR (300 MHz, CDCl₃): δ = 7.50 (d, 1 H, J = 8.8 Hz), 7.50-7.22 (m, 6 H), 7.10 (dd, 1 H, J = 2.5, 8.8 Hz), 5.67 (dd, 1 H, J = 3.2, 3.3 Hz), 3.78 (s, 3 H), 3.18-2.80 (m, 2 H). ¹³ C NMR (75 MHz, CDCl₃): δ = 186.7, 158.1, 138.3, 134.0, 131.8, 129.5, 129.4, 129.3, 129.3, 126.9, 124.9, 121.3, 104.0, 83.4, 56.0, 44.4. Anal. Calcd for C₁₈H₁₄O₂S: C, 73.40; H, 4.76. Found: C, 73.50; H, 4.80.

8

Compound 8: Prepared in the same way as before except crotonaldehyde was used instead of aromatic aldehyde. Yellowish gummy liquid, yield 40%. ¹ H NMR (300 MHz, CDCl₃): δ = 7.75 (dd, 2 H, J = 8.9 Hz), 7.19 (dd, 1 H, J = 2.1 Hz), 6.99 (dd, 1 H, J = 2.1, 8.8 Hz), 4.88-4.84 (m, 1 H), 3.89 (s, 3 H), 2.80-2.60 (m, 1 H), 1.61 (d, 3 H). ¹³ C NMR (75 MHz, CDCl₃): δ = 154.6, 140.2, 134.7, 133.8, 132.1, 125.7, 124.5, 122.5, 111.9, 108.9, 5, 29, 2. Anal. Calcd for C₁₈H₁₂O₂S: C, 67.24; H, 5.17. Found: C, 67.30; H, 5.40.