Abstract
3-(Trimethylsilyl)propyne iminium salts 6a -c and 2-(trimethylsilyl)ethynyl-1,3-benzothiazolium triflate 12 are suitable dienophiles for Diels-Alder reactions with buta-1,3-dienes, cyclopentadiene, cyclohexa-1,3-diene, and anthracene. Tri-tert -butylazete reacts with propyne iminium salts 6a ,b to form Dewarpyridines 15a ,b . Thermal isomerization of 15b generates the sterically overcrowded pentasubstituted pyridine 16 which features a boat-shaped pyridine ring with the highest deviation from planarity so far reported.
Key words
alkynes - iminium salts - Diels-Alder reactions - azacyclobutadiene - non-planar pyridine
References
1 Maas G.
Singer B.
Wald P.
Gimmy M.
Chem. Ber.
1988,
121:
1847
2a Viehe HG.
Baum JS.
Chimia
1975,
29:
300
2b Baum JS.
Viehe HG.
J. Org. Chem.
1976,
41:
183
3 François J.-P.
Gittos MW.
Synth. Commun.
1979,
9:
741
4 Kantlehner W.
Lehmann H.
Stahl T.
Kaim W.
Chem.- Ztg.
1991,
115:
183
5 Rück-Braun K.
Kühn J.
Chem. Ber.
1996,
129:
1057
6a Dötz KH.
Kuhn W.
J. Organomet. Chem.
1985,
286:
C23
6b Rahm A.
Wulff WD.
Rheingold AL.
Organometallics
1993,
12:
597
7 A values (10.5 kJ/mol for SiMe3 , 19.7-20.5 kJ/mol for CMe3 ) which reflect the preference of the equatorial over the axial position at the cyclohexane ring: Eliel EL.
Wilen SH.
Mander LN.
Stereochemistry of Organic Compounds
Wiley;
New York:
1994.
p.690-700
8 Schlegel J.
Maas G.
Synthesis
1999,
100
9 According to PM3 calculations, the orbital coefficient in the LUMO of 6b is higher at the SiMe3 substituted carbon atom of the triple bond (-0.32 vs. 0.05). In the HOMO of isoprene, the orbital coefficient at C-1 is larger than at C-4 (0.59 vs. -0.45).
10 Alder K.
Rickert HF.
Liebigs Ann. Chem.
1936,
524:
180
11 Vogelbacher U.-J.
Regitz M.
Mynott R.
Angew. Chem., Int. Ed. Engl.
1986,
25:
842 ; Angew. Chem. , 1986 , 98 , 835
12 Rahm R.
Maas G.
Chem. Ber.
1994,
127:
1295
13 Regitz M.
Heydt H.
Bergsträßer U. In Advances in Strain in Organic Chemistry
Vol. 5:
Halton B.
JAI Press;
Greenwich:
1996.
p.215-238 ; and references cited therein
14 Ledermann M.
Regitz M.
Angermund K.
Binger P.
Krüger C.
Mynott R.
Gleiter R.
Hyla-Kryspin I.
Angew. Chem., Int. Ed. Engl.
1988,
27:
1599 ; Angew. Chem. , 1988 , 100 , 1616
15 Maas G.
Regitz M.
Rahm R.
Schneider J.
Stang PJ.
Crittell CM.
J. Chem Soc., Chem. Commun.
1990,
1456
16a Vogelbacher U.-J.
Ledermann M.
Schach T.
Michels G.
Hees U.
Regitz M.
Angew. Chem., Int. Ed. Engl.
1988,
27:
272 ; Angew. Chem. ; 1988 , 100 : 304
16b Hees U.
Vogelbacher U.-J.
Michels G.
Regitz M.
Tetrahedron
1989,
45:
3115
17 Maas G.
Fink J.
Wingert H.
Blatter K.
Regitz M.
Chem. Ber.
1987,
120:
819
18 Sheldrick GM.
SHELX-97, Program for the Solution and Refinement of Crystal Structures from Diffraction Data
Universität Göttingen;
Göttingen:
1997.
19 Crystallographic data (excluding structure factors) for the structures of 10a and 16 have been deposited with the Cambridge Crystallographic Data Centre [CCDC-173956(10a ) and -173955(16 )]. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [UK. Fax: +44(1223)336033; E-mail: deposit@ccdc.cam.ac.uk).
