It's often very difficult for an unknown, un-proven startup to successfully sell into the largest companies in your chosen target market. Occasionally, there are exceptions (usually predicated on an unusually strong, senior-level relationship into that account, or high-profile reputation of founders or investors), but the requirements and procurement processes of large organizations are extremely challenging for a small company. Further, trying to sell into such organizations out-of-the-gate can consume precious resources and distract you from getting traction with smaller, more viable early adopters. One of the biggest things more risk-averse large companies will look for is proof with successful, referenceable customers. So generally best to earn those first, then go up-market.