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The use of atmospheric pressure ionization at -78 °C for the analysis of a thermally unstable compound, 2,5dimethylene-2,5-dihydrothiophene, dissolved in methylene chloride is demonstrated with a home-built lowtemperature electrospray source connected to a quadruple mass analyzer. To prevent the analyte molecule from polymerizing at elevated temperatures, the sample solution was maintained at -78 °C in the dry-ice-cooled sample injection and sample loop units. The solution was delivered to the spray needle by compressing the nitrogen gas in a syringe which was connected to the sample loop by a Teflon tube. At such low temperature, a strong ion signal was obtained from the analyte molecule as a result of corona discharge ionization.Electrospray ionization (ESI) mass spectrometry has been proven to be very useful for analyzing not only large biomolecules 1,2 but also small organic and inorganic compounds. [3][4][5][6][7] Since most organic and inorganic compounds do not dissolve well in water, organic solvents such as n-hexane, benzene, chloroform, methylene chloride, acetonitrile, and methanol are used alone or mixed together with water for the analysis of such compounds by ESI. [8][9][10][11][12][13][14][15] Most organic solvents freeze at temperatures far below 0 °C. For example, the melting point of methylene chloride (CH 2 -

Flash vacuum pyrolyses (FVP) of benzoic 3-methyl-2-benzofurancarboxylic anhydride (12) and benzoic 2-methyl-3-benzofurancarboxylic anhydride (13) at 550 degrees C and ca. 10(-2) torr both give methylenebenzocyclobutenone (21) as the major product and indenone (22) as the minor one. A mechanism involving generation of alpha-oxo-o-quinodimethane 11 as the primary pyrolysis product from FVP of 13, followed by elimination of a CO molecule to give carbene 24, which undergoes a vinylcarbene-cyclopropene rearrangement and a ring contraction of the resulting carbene 23, is proposed to account for the observed results. The proposed mechanism is further supported by a deuterium-labeling study on FVP of (2-benzofuryl)methyl-alpha,alpha-d2 benzoate (28-d2).

2,5‐Dimethylene‐2,5‐dihydrofuran (3), generated flash pyrolitically, underwent di‐ and trimerization reactions to give cyclic dimer and trimer. Compound 3 was trapped with methanol, ethanol, bromine, acetic acid, thiophenol and oxygen. Results from trapping experiments suggest that the diradical form of 3 is involved.