J . Org. Chem. 1987, 52, 4062-4072
4062
than the 4aS-CSbl cation; however, only qualitative comparisons can be made. In the 5aT-OSbl cation the maximum a-spin density resides on C4. The slightly longer N-N distance than a normal N=N distance is accommodated by the resonance-contributing structures shown as 14 which is
14
+ 15
equivalent to the MO description in 15 in which the (n + n) MO is doubly occupied in the (n - n) MO is singly occupied. In the 5aT-OSa2 cation the a-electron spin density resides predominantly on C3 and C5. The N-N distance is intermediate between that of N=N and N=N. This cation is best represented by the resonance-contributing structures shown as 16, of the MO description shown in 17.
16
17
Six *-Electron, Open-Shell Cations. Calculations have also been carried on the six a-electron, a-triplet, and singlet open-shell cationic systems. The total energies are given in Table 111, and the a and nonbonded MO energies are given in Table IV. The 3-21G optimized structural parameters, a and total charge densities, and spin densities are given in the structure in Figure 2. The 6-31G optimized structural parameters are given in Table 2 of the supplementary material. The structure and a charge distribution of the 6aT-OS cation suggest that the structure is best
represented by the resonance structures shown as 18. The +N-N+
t
t
+N-N+
t
t
+N-N+
t
t
18
structure of the 6aS-OS cation is quite similar to that of the triplet structure except for a slightly longer N-N distance. Its overall electronic structure is similar to that shown as the hybrid 18. Both the triplet and singlet structures are slightly higher in energy that the 4aS-CSbl cation. Summary Of the closed-shell singlet cationic structures, the 4aSOBbl cation is the lowest in energy. The electronic properties predicted from the calculations correspond to those observed experimentally. The calculations indicate that several other singlet and triplet electronic configurations lie close in energy. The highest lying configuration appear to be the aromatic six a-electron closed-shell system. Acknowledgment. We thank the Computing Center of the University of Notre Dame and Professor John Dow or the Department of Physics for computing time; the latter facility is supported by the Office of Naval Research (Contract No. ONR N0014-84-K-0352). Registry No. 1, 61355-01-9; 2, 109391-18-6; 3, 53283-24-2; 4, 109391-19-7; 5 , 24100-66-1; 6, 5363-19-9; 7, 109391-21-1; 12, 109391-20-0. Supplementary Material Available: Tables of calculated structural parameters for the pyrazole cations studied (7 pages). Ordering information is given on any current masthead page.
Stereoselective Intramolecular Iodoetherification of I-Pentene-1,3-diols: Synthesis of cis -2-(Iodomethyl)-3-hydroxytetrahydrofurans Yoshinao Tamaru, Makoto Hojo, Shin-ichi Kawamura, Shuji Sawada, and Zen-ichi Yoshida* Department of Synthetic Chemistry, Kyoto University, Yoshida, Kyoto 606, Japan Received April 29, 1987
Intramolecular iodoetherification of 4-pentene-1,3-diol and its monosubstituted derivatives, irrespective of the substitution pattern, provides cis-2-(iodomethyl)-3-hydroxytetrahydrofuransin high yields. The cis selectivity generally exceeds 95%. Some of the disubstituted 4-pentene-1,3-diolssimilarly undergo the selective cyclization; however, the others show anomalies on regio- and/or stereoselectivities, providing either tetrahydropyrans or trans-2-(iodomethyl)-3-hydroxytetrahydrofurans as main products. A mechanism that may reconcile all of these results is proposed.
The control of stereochemistry in an acyclic system is an important consideration in the synthesis of many complex molecules.' However, despite the great advances that have been made in cyclic systems, the control of stereochemistry in acyclic systems remains a challenge. The hydroxy group of allylic alcohols often provides a moderate to strong stereo-directing effect during addition to double et^.^). bonds (epoxidation,2 gly~olation,~ hal~genation,~ (1)(a) Bartlett, P. A. Tetrahedron 1980,36,2. (b) Bartlett, P.A. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic: New York, 1984; Val. 3,Chapters 5 and 6.
Katzenellenbogen? Chamberlin,7 and we8 have demonstrated that the hydroxyl group of 3-hydroxy-4-penten(2) (a) Sharpless, K. B. Aldrichimica Acta 1979,12, 63. (b) Rossiter, B. E.; Verhoeven, T. R.; Sharpless, K. B. Tetrahedron Lett. 1979,4733. (c) Chautemps, P.;Pierre, J.-L. Tetrahedron 1976,32, 549 (d) Adams, C. E.; Walker, F. J.; Sharpless, K. B. J. Org. Chem. 1985,50,420. (3) (a) Cha, J. K.; Christ, W. J.; Kishi, Y. Tetrahedron Lett. 1983,24, 3943,3947; Tetrahedron 1984,40,2247. (b) Stork, G.;Kahn, M. Ibid. 1983,24, 3951. (c) Danishefsky, S. J.; Larson, E.; Springer, J. P. J. Am. Chem. Soc. 1985,107, 1274. (d) Vedejs, E.;McClure, C. K. Ibid. 1986, 108, 1094. (4) (a) Santelli, M.; Viala, J. Tetrahedron Lett. 1977,4397.(b) Midland, M. M.; Halterman, R. L. J . Org.Chem. 1981,46, 1227.
0022-326318711952-4062$01.50/00 1987 American Chemical Society
J. Org. Chem., Vol. 52, No. 18, 1987 4063
Stereoselective Iodoetherification of 4-Pentene-1,3-diols amides and 3-hydroxy-4-pentenoicacid plays an important role to effect the selective cyclization and provides cis-3hydroxy-4-(halomethyl)-y-butyrolact~nes. The cis-directed cyclization by the allylic hydroxyl group is general, and wide application is found for the cyclization of 4-pentene-1,3-diolsgand 3-hydroxy-4-pentenylaminesl0 by combination with a variety of electrophiles (e.g., halogens, m-chloroperbenzoic acid, Pd(II), Hg(II), etc.). In this paper, we describe the full account of the systematic investigation on the intramolecular iodoetherification of 4-pentene-1,3-diols, reported previously as a preliminary communication (eq 1).l1 These reactions
Table I. Iodoetherification of Monosubstituted I-Pentene-l.3-diols 1"
entry 1 2
3 4 5 6 7
8
R syn
1
9 10 11 12 13 14 15 16 17 18
R'
2 R'anti
R 2 s y n&R5 I,,,
OH ,,,,A3
1: R b a:R=H a:R=H b: R4 = Me c: R4 = i-Pr d: R2 = (CH2)5 d: R2 = (CH,), d: R2 = (CH2)5 e: RIs,.,, = P h h f: Rlanti= P h g: Rt8,.,, = t-Buh h: R anti = t-BU i: R' = CH2C0,Eth j: R I Z i = CH2C02Et k: RzEyn= P h 1: RPmti= P h m: R2,,,,, = t-Buh n: RPmt,= t-Buh 0: RZ = (CH2)5, R5 = Me
reactn condn:' temp ("C), time (h) A: 0, 3 B: -78, 2 A: 0, 3 A: 0, 6 A: 0, 2 B: -78, 3 c : 0, 1 A: 0, 12 A: 20, 24 A: 0, 18 A: 25, 60 A: 0, 4 A: 25, 30 A: 0, 6 A: 0, 5 A: 0, 70 min A: 0, 70 min B: -78, 2
2:3 ratiod (isol yield, %)
95:5 (87) 92:8 (89) 91:9 (94) >95:95:95:95:95:95:95:95:
