Los Altos-based Box is closing in on a monster lease at 222 2nd (it's probably a big tribute to Leo Durocher), Tishman's 452k SF masterpiece coming to the southwest corner of Howard and Second, multiple sources around town tell us.

Tishman managing director of leasing Allen Palmer said he can't comment. Currently HQ'd in 100k SF at 4440 El Camino Real, the cloud storage giant also leases 20k SF in the city at 100 First. Last fall, Neustar inked 144k SF to take half of Tishman's 505 Howard as the anchor tenant, helping move forward this next office tower. The project has been years in the making: The site was purchased in 2006 and received full entitlements in December 2010. (That's a bit early for a debutante, but when you're ready, you're ready.) The Thomas Phifer-designed tower will have a steel and concrete frame with a glass curtain wall and house 447k SF of office space, 5k SF of retail, and 8,500 SF of public space.

There's already been interest around town for 222; sources tell us law firm Fenwick, currently at 555 Cal, had an LOI but that deal is off the table now. Box's big transaction would be significant because its arch rival Dropbox would sit about half a mile away. (If Box were to take all of 222 2nd, it would quickly catch up to Dropbox's signed footprint in S.F., which totals some half a million square feet.) Last month Dropbox opted to take not one, but two, new neighboring buildings rising in SoMa: all 100k SF of Breevast's five-story building at 345 Brannan (rendered above) as well as Kilroy's 180k SF 333 Brannan.