C. Wentrup, W. Heilmayer, and G. Kollenz, Science of Synthesis: Houben-Weyl Methods of Molecular Transformations, vol.23, pp.1219-1248, 1994.

. Thieme, . P. Stuttgart-;-c)-k, S. D. Reber, E. J. Tilley, ). A. Sorensen-;-d et al., For representative recent work: e), vol.38, pp.4124-4127, 2006.

). J. Galvez, J. Castillo, J. Quiroga, M. Rajzmann, J. Rodriguez et al., Adv. Synth. Catal, vol.16, pp.3010-3013, 2014.

). J. Hyatt, P. L. Feldman, and R. J. Clemens, Acc. Chem. Res, vol.49, pp.628-642, 1984.

W. Kirmse, 2193-2256; b) Y. Coquerel, J. Rodriguez in Molecular Rearrangements in Organic Synthesis, Eur. J. Org. Chem, issue.3, pp.59-84, 2002.

). G. Wittig, A. Haag, C. Ber, ). Bestmann, H. Hartung et al., Beilstein J. Org. Chem, vol.96, pp.1494-1495, 1963.

T. Yoshino, F. Ng, S. J. Danishefsky, ;. Boisse, B. Rigo et al., For representative syntheses, vol.128, pp.4206-4209, 2006.

H. Strzelecka, M. Simalty-siemiatycki, C. Prÿvost, C. R. Hebd-;-m.-von-strandtmann, M. P. Cohen et al., For the previous reports describing isolated examples of an "abnormal Wittig olefination, vol.257, pp.5069-5072, 1963.

A. K. Miller, D. Trauner-;-b, ). W. Wilk, H. Waldmann, and M. Kaiser, Bioorg. Med. Chem, vol.17, pp.2304-2309, 2006.

K. J. Sharma, J. Powell, A. S. Burnley, J. E. Awaad, ;. Moses et al., For recent total syntheses: d), vol.81, pp.5190-5201, 2011.

). L. Hoffmeister, T. Fukada, G. Pototschnig, A. C. F¸rstner-;-f)-j, T. K. Henrikson et al., ACS Chem. Biol, vol.21, pp.1241-1252, 1959.

, For approaches to di-or trisubstituted g-pyrones involving inverse electron demand hetero Diels-Alder reactions with a-oxoketenes

J. J?ger, L. Wenzelburger, . Ann, ). E. Chem-;-b, T. P. Wenkert et al., For an approach to a'-methoxy-g-pyrones based on a desymmetrization strategy, J. Org. Chem, vol.55, pp.9429-9437, 1976.

L. Zhong, Y. Wang, B. Ye, Y. Song, ). S. Liang-;-g et al., Selected miscellaneous approaches: h), Tetrahedron Lett, vol.78, pp.123-124, 2005.

G. P. Morris, D. G. Luke, and . Wishka, J. Org. Chem, vol.61, pp.3218-3220, 1996.

C. A. Rodrigues, A. Misale, F. Schiel, and N. Maulide, Org. Biomol. Chem, vol.15, pp.1596-1600, 2017.

A. B. Marco, P. M. Burrezo, L. Mosteo, S. Franco, J. Garìn et al., , vol.5, pp.231-242, 2015.

:. R. Review, V. J. Pratapand, and . Ram, Tetrahedron, vol.73, pp.2529-2590, 2017.

K. Minami, T. Shirahata, and M. Yohji, For examples of Wittig-type olefinations of 4H-pyran-4-one derivatives, vol.42, pp.1694-1699, 2013.

). R. Robiette, J. Richardson, V. K. Aggarwal, J. N. Harvey-;-p, D. G. Byrne et al., Review on the Wittig olefination mechanism: b), vol.128, pp.6670-6696, 2006.

E. P. Couzijn, J. C. Slootweg, A. W. Ehlers, and K. Lammertsma, J. Am. Chem. Soc, vol.132, pp.18127-18140, 2010.

, See Supporting Information for details. It may be noted that the present study is the first realistic computational mechanistic study on the Wittig reaction with ketenes

, Chem. Eur. J, vol.24, pp.11110-11118, 2018.

E. L. Clennan and P. C. Heah, J. Org. Chem, vol.47, pp.3329-3331, 1982.

, with the pre-exponential factor of the Eyring equation assumed to be 1), which appears over-estimated. Also, an overall barrier of 159.7 kJ mol?1mol?1 would be involved in the reaction 4 h!5 a, which also appears over-estimated. Given the large approximations of the model employed, relative errors of up to ? 40 kJ mol?1mol?1 are undoubtedly possible. For an excellent discussion on the accuracy of computational models in the case of polar reactions in solution such as the Wittig olefination, see, Faraday Discuss, vol.145, pp.487-505, 2010.

X. Bugaut, D. Bonne, Y. Coquerel, J. Rodriguez, and T. Constantieux, Curr. Org. Chem, vol.17, 1920.

). D. Bonne, T. Constantieux, Y. Coquerel, J. Rodriguez-;-c, ). D. Bonne et al., Stereoselective Organocatalysis: Bond Formation Methodologies and Activation Modes, vol.10, pp.559-585, 2010.

, Chem. Eur. J, vol.24, pp.11110-11118, 2018.