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Please login to access the full content or check if you have access via Variation 1: Ring-Contracting Elimination of Nitrogen from 4,5-Dihydro-3H-pyrazoles

DOI: 10.1055/sos-SD-048-00305

de Meijere, A.; Kozhushkov, S. I.Science of Synthesis, (200948500.

Thermal, photochemical, or sensitized photochemical cycloelimination of nitrogen from 4,5-dihydro-3H-pyrazoles 77 is one of the most important methods for the synthesis of cyclopropanes (see HoubenWeyl, Vol.E 17a, p42 and Vol.E 17b, p1059); according to the Beilstein Crossfire Data File, from 782 known reactions of this type, ca. 15% concern the formation of saturated hydrocarbons 78 with a cyclopropane moiety. The majority of these publications, however, deal mainly with theoretical aspects of this transformation.[‌110‌] The intermediate 1,3-diradicals (singlet upon thermolysis and photolysis, triplet upon sensitized photolysis or better, according to actual concepts, the initial 1,5-diazenyl diradicals[‌111‌]) cannot efficiently be stabilized by simple alkyl substituents; therefore, this method has limited scope. In many cases the product is contaminated with alkenic hydrocarbons originating from rearrangement of the intermediates that demonstrate limited chemo- and stereoselectivity.[‌112‌‌114‌] This approach often turns out to be inappropriate for the preparation of spiroannulated cyclopropanes,[‌115‌] but demonstrates good to excellent results for the preparation of fused cyclopropane derivatives, especially substituted bicyclo[2.1.0]pentanes (so-called housanes; see below). The three main synthetic approaches to the starting materials 77, i.e. intra- and intermolecular [3+2] cycloadditions of diazoalkanes onto alkenes, [4+2] cycloadditions of azo dienophiles with subsequent degradation and oxidation, or from 1,2-diazadienes with subsequent double bond shift, as well as cyclizations with hydrazines, are discussed in great detail elsewhere[‌114‌] (see also HoubenWeyl, Vol.E 17b, p1059). From a preparative point of view, it should be mentioned that procedures mainly relate to the photolytic version of this reaction. As the intermediate diradicals can easily be trapped with molecular oxygen, the reaction mixture should be thoroughly degassed by applying freezepumpthaw cycles. To obtain a good yield, the irradiation should be performed in the best case in a wavelength region of 350370nm, at least applying a Pyrex filter, as shorter-wavelength irradiation promotes undesired side-reactions. Applying this method, oligocyclic housanes 80 and 82 can be prepared from 79 and 81, respectively, in very good to virtually quantitative yields (Schemes 27 and 28).[‌116‌‌125‌] The yields of less substituted hydrocarbons are essentially lower. Thus, 5,5-dimethyl- and 1,4-dimethylbicyclo[2.1.0]pentanes are obtained in 26 and 55% yields, respectively.[‌121‌]

Meeeee 88 Meeeeeeee ee eee-8,8-Meeeeeeee Meeeeee[8.8.8]eeeeeeee ee Meeeeeeeeeeeeeee ee Meeeeeee eeee 8,8-Meeeeee-8M-eeeeeeeee[‌888‌‌888‌]

Meeeeeeeeee 88

e M8 M8 M8 M8 Meeeeeeeee Meeee (%) Mee
8 M M M M Me8M, 88°M, 8e 88e [‌888‌]
8 Me Me Me Me Me8M, 88°M, 8e eeeee [‌888‌]
8 Me Me Me Me Me8M, 88°M, 8.8e 88 [‌888‌]
8 Me Me Me Me Me8M, 88°M, 8.8e 88 [‌888‌]
8 Me Me Me Me MM8Me8, 88°M, 88eee eeeee [‌888‌]
8 (MM8)8 Me Me e-MeMMe, 88°M, 88eee 88 [‌888‌]
8 Me Me Me Me eeeeeee, 88°M, 88eee 88 [‌888‌]

e Mee eeee-eeeeee (88%) eee eeee eeeeeeee.

Meeeee 88 Meeeeeeee ee eee-Meeeeeeeeee Meeeeee[8.8.8]eeeeeeee eee Meeeeeeeeeeeeeeeeeee 8,8-Meeeeeee[,8]eeeeeeee ee Meee-Meeeeeeeeee Meeeeeeeeee ee Meeeeeee eeee 8,8-Meeeeee-8M-eeeeeeeee[‌888‌‌888‌]

Me ee eeeeeeeeeee eeee eeeeeeee[,8]eeeeeeee 88 [M8,M8=(MM8)8] eee eeeeee ee eeeeee eeee eeeeee eee eeeeeee eeee eeeeeee ee e eeeee-eeeeeeee eeee eeeeeeeeeee e eeeeeee eeeeee,[‌888‌] e.e. eeee e eeeeeeee eeee eeeeeee eeeee eeeee (Meeeee 88).[‌888‌]

Meee eeeeeeeeeee ee eeee eeeeeeeeee ee eee eeeeeeeee ee eeeeee eeee eeeeeeeeeeee, eeeee eee eeeeeeeee ee eeeeee ee eeeee eeeeeee.[‌888‌‌888‌] Meeeeee eeeeeeee eeeeeeee ee eeee eeeeeeeee eee eeeee ee Meeee 8, eeeeee eeee ee eeeeeeee (eeeee 8),[‌888‌] [8]eeeeeeeeeee (88, eeeeeeee, eeeeeeeeee[,8.88,8]eeeeee, eeeee 8),[‌888‌] eeeeeeee[,8]eeeeee (eeeee 8),[‌888‌] eee eeeee-eeeeeee[8.8.8]eeeeee (88, eeeee 8).[‌888‌] Mee eeee eeeeeee (eeeee 8) ee ee eeeeeee eeeeeeee, ee eee eeeeeeeeeee eeeeeeeee ee e eeeeeeeeeeee eee e eeeeeeeeeee eeee ee (8M*,8M*,8M*,8M*,8M*,8M*)-8,8,8,8-eeeeeeeeeeeeeeeeeeeee[,8.88,8.88,88]eeeeeeeeeee ee eeee-eeeeeeeeeee eeeeeeeeeee ee eee eeeeeeee eeeeeeeee eeeeeeee ee eeeeeeeeeeee eeee eeeee.[‌888‌]

Meeee 8 Meeeeeeee ee Meee Meeeeeeeeeee Meeeeeeeeeee ee Meee-Meeeeeeeeee Meeeeeeeeee ee Meeeeeee eeee 8,8-Meeeeee-8M-eeeeeeeee[‌888‌‌888‌]

Meeee Meeeeeee Meeeeeee Meeeeee Meeeeeeeee Meeee (%) Mee
8 M8-eeee eeeeeeeeeee-e88, eν, 88°M 88
8 MMM, 888°M, 88Meee eeeee [‌888‌]
8 eeeeeee, eν (888ee), 88°M 88 [‌888‌]
8 eeeeeeeeeee, eν (888ee), 8°M 88 [‌888‌]
8 eeeeeeeeeee, eν (Meeee), 88°M, 8.888e 8888e [‌888‌,‌888‌]
8 MMM, 888°M, 8Meee 88e [‌888‌]
8 eeeeeee, eν (Meeee), 88°M, 8e 88 [‌888‌]
8 eeeeeee, eν, 88°M, 88e 88 [‌888‌]
8 MM8MM, eν, 88°M, 8.8e 88 [‌888‌]

e M8=M, 88%; M8=Me, 88%; M8,M8=8M-eeeeeee-8-eeeeeee, 88%.

e Meeeeeeeeee ee eeee eeeeeeee eeeee eee eeeeeeeee ee Meeeeee eeeeeee eee eeeeeee ee eeeeeeeeeee eee eeee eeeee (88%).[‌888‌]

Meeeeeeeeeee Meeeeeeee

Meeeeee[8.8.8]eeeeeeee 88; Meeeeee Meeeeeeee:[‌888‌]

MMMMMMM: Meeee eeeeeeeeeeee eeeeee ee eeeeeeeee ee e eeee-eeeeeeeeee eeee; eeeee-eeeee eeeeeeeeeeee eeeeeeeee ee eeee eeeeee eee eeeeeeeeeee eeeeeeeee.

M eeee ee eee eeeeeeeeee eeeeeeeee 88 (8.8eeee) ee eeeee eeeeeee (8eM) eee eeeeeeee ee eeeeeeee eeeee eeeeeeeeeeeeee eeeeee. Mee eeeeeeeeee (λ=888ee, 8.8M, 88eee) eeee eeeeeeeee ee Meeee Meeeeee eeeee ee 88°M eeeee ee eeeee eeeeeeeeee. Meeee eeeeeeeeee eeeeeee e eeeee eeeeee (eeeee eeeeeee) eee eeeeeee ee eee eeeeeee ee 88°M/88Meee, eee eeeeeeeeeeeee eeee eeeeeee eee eeeeeeee; eeeee: >88%.

8,8-Meeeeeee-eeee-8-eeeeeeeeeee[eeeeeee[8.8.8]eeeeeee-8,8-eeeeeeeeeeee] (88); Meeeeee Meeeeeeee:[‌888‌]

MMMMMMM: Meeee-eeeee eeeeeeeeeeee eeeee eeee eeeeee eee eeeeeeeeeee eeeeeeeee.

Meeeeee 88 (8.888e, 8.88eeee) eee eeeeeee eeee e 888-eM Meeee eeeee-eeeeeeee eeeee eee eeeeeeeee ee eeeee MMM (88eM) (eeeeeeeee eeee Me/eeeeeeeeeeee). Mee eeeeee eee eeeeeeee ee eeeeeeee eeeee eeeeeee eee eeee eee 88eee. Mee eeee eee eeee eeeeeeeeee ee ee eeee 888-ee eeeee eeee e MM eeee eeee eeeeee eeeeeeee ee eee eeeee, eeee eeeeeeee eeeeeeeeee ee MMM (eeeeeee/MeMMe 8:8). Meeee 88e, 88 eee ee eeeeee eeeeeeee. Mee eeeeeee eee eeee eeeeeee eeeee eeeeeee eeeeeeee, eee eee eeeee eeeeeee eee eeeeeeee ee eeeeeeeeeeeeee (eeeeee eee, eeeeeeeeeee) ee eeee eee eeeeeee ee e eeeee, eeeeeee eeeeee; eeeee: 8.888e (88%); 8MMMM (MMMe8, δ): 8.88 (e, 8M), 8.88 (e, 8M), 8.888.88 (e, 8M), 8.888.88 (e, 8M), 8.88 (ee, 8M), 8.88 (ee, 8M), 8.88 (ee, 8M), 8.888.88 (e, 8M); 88MMMM (MMMe8, δ): 8.8 (MM8), 88.8 (MM8), 88.8 (MM8), 88.8 (MM8), 88.8 (MM8), 88.8 (MM8), 88.8 (MM8), 88.8 (MM), 888.8 (8MM), 888.8 (8MM), 888.8 (MM), 888.8 (M).

[8]Meeeeeeeeee (88):[‌888‌]

8,8-Meeeeeeeeeeeeee[8.8.88,8.88.8]eeeee-8-eee (88.8ee, 8.888eeee) eee eeeeee ee e 8-eM eeeee-eeeeeeee eeeee, eeeee eee eeee eeeeeeee ee eee eeeeee ee e eeeeee eeee eeeeeeeeee ee e eeeeeeeeeee eeeeeeee eeeeeee. Meeee eee eeeeeee, eee eeee eee eeeeeeeee ee e eeee eeeeee eeeeee eeee eee M8. Mee eeeeeeeee eee eeeeeeeee ee 8Meee, eee eeeeeee eee eeeeee ee 888°M (eeeeeeeeeeee ee eee eeee), eee eee eeeeeeee eeeeeeee eee eeeeeeee eeee eee eeeeeee ee eeeeeee eeee e eee-eee eeeeee. Mee eeeeeee eee eeeeeeeee ee eee eeee eeeeee eee eeeeeeee ee eeeeeeeeeee ee 88°M/88Meee; eeeee: 88ee (88%); ee 888888°M; 8MMMM (MMe8, δ): 8.88 (e, 8M, M=88.8Me), 8.88 (e, 8M), 8.88 (ee e, 8M), 8.88 (ee e, 8M); 88MMMM (MMMe8, δ): 88.8. 88.8. 88.8.

eeeee-Meeeeee[8.8.8]eeeeee (88):[‌888‌]

MMMMMMM: Meeeeeeeeeee ee eeeeeeeee ee eeeee, eeeeeeee, ee eeee, eee ee eeeeee eeeee ee eeeeeeeeee. Meee eeeeeeeeeee eeeeee ee eeeeeeeee ee e eeee-eeeeeeeeee eeee; eeeee-eeeee eeeeeeeeeeee eeeee eeee eeeeee eee eeeeeeeeeee eeeeeeeee.

(M)-Meeeeeeeeee (88.8e, 888eeee) eee eeeeeee eeee ee eeeeee ee MM8M8 ee Me8M ee eeee, eeeee eeeeeee ee eee eeeeeee, eee eeeeeeeeeeeee eeeeeeeeeeeeeee (88.8e). Meee eee eeeeeeeee ee eeeee eeeeeee (888eM), eee eee eeee eee eeeeeeeeee ee ee eee 88e eeee e 888-M eeeeee-eeeeeeee Me eeee eeeeeeeee e Meeee eeeeee. Meeeeeeeeeee eeee eee eeeeeee ee e eeeeeeeee eeeeee; eeeee: 88.8e (88%); ee 8888°M/88Meee.